Topic 4 review
Name _____________Key________________
1. What is the relationship between genes and chromosomes?
gene—segment of DNA
--responsible for production of 1 protein
--Mendel’s factor
Genes are found on chromosomes
Each gene has a specific location called a locus on a certain chromosome
2. What is a locus?
locus—specific position of a gene on a chromosome
3. Differentiate between gamete and somatic cell.
Somatic cells (nonreproductive cells) have two sets of chromosomes; produced by mitosis; genetically identical
Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells; produced by meiosis;
genetically unique
4. What are the two human gametes? Egg and sperm
5. Differentiate between sexual and asexual reproduction.
In asexual reproduction, one parent produces genetically identical offspring by mitosis. a single
individual is the sole parent and passes copies of all its genes to its offspring. As a result, the offspring are
an exact copy of themselves (a clone).
• Any genetic difference would be due to mutation
• Does not involve the formation of gametes (sex cells)
In sexual reproduction, two parents give rise to offspring that have unique combinations of genes
inherited from the two parents - Involves the formation of gametes
6. What constitutes the life cycle of an organism?
Life Cycle is the sequence of stages in the reproductive history of an organism from conception to the
production of its own offspring
7. What are homologous chromosomes?
The two chromosomes in each pair are called homologous chromosomes, or homolog. Homologous
chromosomes have the same length, centromere position, staining pattern and carry genes controlling
the same inherited characters. The sex chromosomes are X and Y and are also known as allosomes.
8. How are they arranged on a karyotype? How many pairs? How many total? What is the 23rd pair?
A karyotype is an ordered display of the pairs of chromosomes from a cell (usually a wbc) in metaphase
• Karyotype- a display of the chromosome pairs of a cell arranged by size and shape – largest to
smallest. 23 pairs.
– 22 pairs of autosomes (42 total chromosomes)
– 1 pair of sex chromosomes (23rd pair- 2 chromosomes)
9. What is the purpose of a karyotype?- can determine sex, disease, errors in chromosomes
10. What pair of chromosomes is not homologous? The sex chromosomes are X and Y and are also
known as allosomes.
11. What are autosomes? What are allosomes? The sex chromosomes are X and Y and are also known
as allosomes. Human females have a homologous pair of X chromosomes (XX). Human males have
one X and one Y chromosome. The 22 pairs of chromosomes that do not determine sex are called
autosomes
12. Differentiate between diploid and haploid and how they relate to gametes and somatic cells.
A diploid cell (2n) has two sets of chromosomes. A gamete (sperm or egg) contains a single set of
chromosomes, and is haploid (n). For humans, the haploid number is 23 (n = 23). Each set of 23
consists of 22 autosomes and a single sex chromosome
13. Explain the relationship between fertilization and meiosis. - Fertilization is the union of gametes
(the sperm and the egg). Gametes are the only types of human cells produced by meiosis, rather
than mitosis.
14. What is a zygote? - The fertilized egg is called a zygote and has one set of chromosomes from each
parent
15. Give 3 characteristics of facts about meiosis.
Meiosis results in one set of chromosomes in each gamete. Two Divisions. Crossing over occurs. The two
cell divisions result in four daughter cells, rather than the two daughter cells in mitosis. Each daughter
cell has only half as many chromosomes as the parent cell.
16. What happens during interphase?
i. Precedes meiosis
ii. Chromosomes replicate as in mitosis
iii. In animal cells, centriole pairs duplicate
17. List 5 significant events that occur during prophase I.
-Chromosomes become visible as long, thin, single threads
-Chromosomes begin to contract which continues throughout Prophase I
-homologous chromosomes undergo synapsis (pairing gene by gene)
--involves formation of synaptonemal complex (protein structure that aids in the chromosomal
pairing)
--doubled chromosomes appear as tetrads (refers to doubled chromosomes in Prophase I)
--crossing over occurs
18. Explain the relationship between synapsis and tetrads.
--homologous chromosomes undergo synapsis (pairing gene by gene)
--involves formation of synaptonemal complex (protein structure that aids in the chromosomal
pairing)
--doubled chromosomes appear as tetrads (refers to doubled chromosomes in Prophase I)
19. What is the relationship between tetrads, dyads, and monads?
Tetrads – Four sister chromatids
Dyads – pair of sister chromatids, (half of a tetrad, refers to the chromosomes in Anaphase I)
Monads – one chromatid, half of a dyad,
20. What happens in metaphase I in relation to tetrads? -Tetrads are aligned on the metaphase plate
21. What is the difference in metaphase I and II?
Metaphase I - Tetrads are aligned on the metaphase plate
Metaphase II - Chromosomes (dyads) align on the metaphase plate
22. What is the difference in anaphase I and II?
Anaphase I – Chromosomes move toward the poles by the spindle apparatus. Chromosomes now referred
to as dyads.
Anaphase II - Centromeres of sister chromatids separate. Monads (refers to chromosomes in Anaphase II)
move toward opposite poles
23. What is the difference in telophase I and II?
* Telophase I - Spindle apparatus continues to separate homologous chromosome pairs (dyads) until the
chromosomes reach the poles. Cytokinesis occurs as in mitosis. In some species, the nuclear envelope and
nucleoli reappear, and the daughter cells enter a period of interkinesis before Meiosis II. In other species,
the daughter cells immediately prepare for Meiosis II. NO DNA replication occurs before Meiosis II
* Telophase II - Nuclei form at opposite poles of the cell. Cytokinesis occurs producing four haploid
daughter cells
24. Differentiate between cleavage furrow and cell plate.
i.
ii.
Animal Cells
1. Occurs as cleavage
2. Cleavage furrow forms as a shallow groove on cell surface
near metaphase plate
(in animal cells only)
3. Contractile ring of actin and myosin microfilaments forms on cytoplasmic side of furrow
4. Microfilaments contract until parent cell is pinched in two
Plant Cells
1. Cell plate forms across midline of parent cell (equator)
2. Cell plate forms from fusing vesicles derived from Golgi apparatus that have moved
along microtubules to cell’s center
3. Cell plate enlarges until its surrounding membrane fuses with the existing plasma
membrane
4. New cell wall forms between two membranes from contents of cell plate
25. Give three reasons for genetic variation.
a. Independent assortment of chromosomes
In independent assortment, each pair of chromosomes sorts maternal and paternal homologues
into daughter cells independently of the other pairs
The number of combinations possible when chromosomes assort independently into gametes is
2n, where n is the haploid number
b. Crossing over
c. Random fertilization
26. Explain the process of crossing over. When does it occur? - Crossing over produces recombinant
chromosomes, which combine genes inherited from each parent. Crossing over begins in Prophase
I, as homologous chromosomes pair up gene by gene (synapsis). Synapsis involves the formation
of synaptonemal complex (a protein structure that brings chromosomes into close association). In
crossing over, homologous portions of two nonsister chromatids trade places. Crossing over
contributes to genetic variation by combining DNA from two parents into a single chromosome
27. Distinguish between P, F1, F2.
P = parental generation (parents) – true breeding
F1 = first offspring generation, generation resulting immediately form a cross of the first set of parents
(parental generation) – first filial generation – offspring of P
F2 = Result of a cross between two F1 individuals (from F1 generation) – second filial generation
(offspring of F1)
28. Distinguish dominant from recessive, homozygous from heterozygous, genotype from phenotype.
homozygous—two alleles for a particular characteristic are identical
--TT or tt
heterozygous—two alleles for a particular characteristics are different
--Tt
genotype—actual genetic makeup
--symbols
--Tt, TT, or tt
phenotype—effects of the genes
--description of appearance
--tall or short
dominant—refers to trait that always appear in offspring of parents with contrasting traits
tall x short
|
tall
recessive—refers to trait that does not appear in offspring of parents with contrasting traits
29. What does the law of segregation state?
. Law of Segregation
• The two genes (factors) for each character segregate during gamete formation
• Occurs during meiosis
Law of Independent Assortment –
•
•
•
•
•
States: Each allele pair segregates independently of other gene pairs during gamete
formation.
Developed from dihybrid crosses
Refers to behavior of genes during gamete formation
Refers to genes on different chromosomes.
Ex: RrTt x RrTt
30. What is a testcross? Why is it used?
testcross—cross unknown genotype with a homozygous recessive
--used to attempt to determine if a dominant trait is homozygous or heterozygous
--T_ x tt
31. Distinguish between monohybrid and dihybrid.
monohybrid cross—cross in which only one character is considered
TT x tt
|
Tt (monohybrid)
dihybrid cross—cross in which two characters are considered
TTyy x ttYY
|
TtYy (dihybrid)
32. Distinguish between the rule of multiplication and addition.
Multiplication
i. the probability that independent events will occur simultaneously is the product of
their individual probabilities
ii. What is the probability that offspring will be pp if the parents are Pp x Pp?
-First determine the probability that an egg will receive a “p” allele (½)
-Then determine the probability that a sperm will receive a “p” allele (½)
-Solution:
½x½=¼
Addition
•
•
The probability of an event that can occur in two or more independent ways is the
sum of the separate probabilities of the different ways.
In the cross Pp x Pp, what is the probability of the offspring being heterozygous
(Pp)?
-2 ways:
egg
P (½)
p (½)
-Solution
¼+¼=½
sperm
probability
p (½) = Pp ¼
P (½) = pP ¼
33. Be able to work complete, codominance, incomplete, multiple allele, and epistasis problems. Also
be able to explain what these words mean.
Complete Dominance
•
The phenotypes of the heterozygote and the dominant homozygote are indistinguishable.
Incomplete Dominance
• Dominant phenotype is not fully expressed in heterozygote, resulting in a third
phenotype that is intermediate between homozygous dominant and homozygous
recessive. neither allele is completely dominant and the F1 hybrids have a
phenotype somewhere between those of the two parental varieties.
Incomplete Example:
P
red flowers x white flowers
F1
pink flowers (self-pollinate)
F2
¼ red ½ pink ¼ white
Fig. 14-10-3
P Generation
Red
R R
CC
White
W W
C C
Gametes
R
C
C
W
Pink
R W
CC
F1 Generation
Gametes 1/2 CR
1
/2 C
W
Sperm
1
R
/2 C
1
W
/2 C
F2 Generation
1
/2 CR
R
R
CC
Eggs
1
R
W
W
W
CC
W
/2 C
R
W
CC
Codominance
C C
•
•
Defined as the full expression of both alleles in a heterozygote
Ex: MN blood type
• Tay Sachs
• At the organismal level, the allele is recessive
• At the biochemical level, the phenotype (i.e., the enzyme activity level) is
incompletely dominant
• At the molecular level, the alleles are codominant
Blood Type
Genotype
M
MM
N
NN
MN
MN
Multiple Alleles
•
•
•
•
•
•
Epistasis
More than two alternative forms (alleles) of a gene
Ex: ABO blood group
The four phenotypes of the ABO blood group (A, B, AB, O) in humans are
determined by three alleles for the enzyme (I) that attaches A or B
carbohydrates to red blood cells: IA, IB, and i.
• 3 alleles produce 4 possible phenotypes—A, B, O, AB
IA and IB are dominant to i (recessive).
IA and IB are codominant to each other.
Each person carries only 2 alleles.
Interaction between two nonallelic genes in which one alters the phenotypic
expression of the other (a gene at one locus alters the phenotypic expression of
a gene at a second locus)
• Dihybrid cross results will deviate from expected ratio of 9:3:3:1
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for color and c for no color) determines whether the pigment
will be deposited in the hair
•
•
•
34. What is pleiotropy?
Pleiotrophy
• Ability of a single gene to have multiple phenotypic effects
• For example, pleiotropic alleles are responsible for the multiple symptoms of
certain hereditary diseases
• Ex: sickle cell anemia, cystic fibrosis
35. What is polygenic inheritance?
i. Additive effect of two or more genes determines a single phenotypic character
ii. Ex: Skin pigmentation in humans is controlled by at least 3 separately inherited
genes.
--incomplete dominance
--AABBCC- very dark
--aabbcc-very light
--AaBbCc-intermediate shade
--phenotype can be affected by environmental factors (sun exposure)
36. What is a pedigree? Be able to draw/work one.
Human Pedigrees (Family Tree)
• Shows inheritance pattern of a particular phenotypic character
• Used to show Mendelian inheritance in humans
• Helps to understand the past and predict the future
• Important in genetic counseling for disabling or lethal disorders
37. List 3 recessive disorders in humans and tell the characteristics of each one.
Recessive disorders - If a recessive allele that causes a disease is rare, then the chance of two
carriers meeting and mating is low
Consanguineous matings (i.e., matings between close relatives) increase the chance of mating
between two carriers of the same rare allele
Cystic Fibrosis
•
•
•
Most common lethal genetic disease in US
Common in Caucasians
Caused by lack of or defective membrane protein that pumps Cl- out of cells
•
•
•
•
•
Tay-Sachs
•
•
•
•
Sickle-cell Anemia
•
•
•
•
•
Albinism
•
Glactosemia
•
PKU
•
•
•
striking one out of every 2,500 people of European descent
Increased secretions of mucus from pancreas and lungs
No cure
Symptoms include mucus buildup in some internal organs and abnormal
absorption of nutrients in the small intestine
Treat with diet and antibiotics
Most common in individuals of Central European Jewish descent
Enzyme does not function properly; can not metabolize gangliosides (lipid)
Lipids accumulate in brain
Lethal gene
Most common in African-Americans
Caused by single amino acid substitution in hemoglobin (valine for glutamic acid)
Causes abnormally shaped rbc
Heterozygous individuals are said to have sickle-cell trait
Heterozygous condition is malaria resistant
Decreased or lack of production of melanin
Lack an enzyme to break down galactose
Phenylketanuria
Can not metabolize amino acid phenylalanine
Can result in mental retardation
38. List 2 dominant disorders in human and tell the characteristics of each one.
Achondroplasia
• Type of dwarfism
• Lethal in homozygous state
Huntington’s Disease
• Degeneration of nervous system
• Late-acting lethal dominant
• Onset about age 40
Manic-Depressive Disease
Polydactyly
• More than 5 digits on hand and/or feet
39. Explain some ways we can do genetic testing.
Carrier Recognition
• Tests are available to detect carriers of Tay-Sachs, cystic fibrosis, and sickle-cell
trait.
Fetal Testing
a. Amniocentesis
• involves removal of amniotic fluid
• can analyze fluid for chemical content
• culture cells for karyotyping
• done between 14-16 weeks
b. Chorionic Villi Sampling
• fetal tissue suctioned from chorionic villi of placenta
• results within 24 hours
c.
d.
e.
F.
• done at 8-10 weeks
Ultrasound
• sound waves used to study fetal structure
Fetoscopy
• involves insertion of fiber-optic scope into uterus
Newborn Screening
• PKU screening
Multifactorial Disorders
• Disease that have both genetic and environmental influences
• Ex: heart disease, diabetes, cancer, alcoholism, some forms of mental illness
40. What is the chromosomal theory of inheritance?
Theory of inheritance is based on these observations. According to this theory:
 Mendelian factors (genes) are located on chromosomes.
 Chromosomes segregate and assort independently during meiosis
41. What is a sex linked gene?
1. The sex chromosomes have genes for many characters unrelated to sex
2. A gene located on either sex chromosome is called a sex-linked gene; however we generally call
those located on the X chromosome as sex-linked genes
3. Genes on the Y chromosome are called holandric genes and are found in males only
4. In humans, sex-linked usually refers to a gene on the larger X chromosome
5. Sex-linked genes follow specific patterns of inheritance
6. For a recessive sex-linked trait to be expressed:
◦ A female needs two copies of the allele
◦ A male needs only one copy of the allele
7. Sex-linked recessive disorders are much more common in males than in females
8. If a sex-linked trait is due to a recessive allele, a female will express the trait only if she is
homozygous.
9. A heterozygous female is a carrier.
10. Males only need 1 allele of a sex-linked trait to show the trait.
11. Males are hemizygous (only one copy of a gene is present in a diploid organism)
12. Fathers pass sex-linked alleles to only and all of their daughters
A
X —Normal gene
Xa—Sex-linked gene
XAXA—Normal Female
XAXa—Carrier Female
XaXa—Affected Female
XAY—Normal Male
XaY—Affected Male
13. Mothers can pass sex-linked alleles to both sons and daughters.
14. If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to
them will have the disorder, regardless of sex.
15. Daughters who do not have the disorder will be carriers, whereas males without the disorder will
be completely free of the recessive allele
42. What are 3 sex linked disorders and what are the characteristics of them?
a. Color Blindness
-recessive
-can’t distinguish certain colors
-red-green most common
b. Duchenne’s Muscular Dystrophy
-recessive
-atrophy of muscle
c. Hemophilia
-recessive
-blood fails to clot because of lack of a clotting factor
43. Who is Thomas Hunt Morgan? What did he do? What did he work with?
Thomas Hunt Morgan (1910)
 Began experiments with Drosophila melanogaster (fruit fly)
 Discovered X-linked (sex linked) inheritance
 Discovered sex determination (X and Y chromosomes)
 Provided convincing evidence that Mendel’s factors are located on chromosomes
 Discovered Sex-linked Traits
 Morgan’s finding of the correlation between a particular trait and an individual’s sex
provided support for the chromosome theory of inheritance
44. Explain the relationship between X inactivation and Barr bodies.
1. Female mammals have only one fully functional X chromosome in diploid cells.
2. Proposed by Mary F. Lyon and known as the Lyon Hypothesis
3. Each of the embryonic cells inactivates one of the two X chromosomes.
4. Inactive X contracts into densely staining object called a Barr Body.
5. Ex:
 Mosaic coloration of calico cats
 Normal sweat gland development in humans
45. What is the relationship between crossing over and linked genes?
Linked Genes –
* Genes located on the same chromosome that tend to be inherited together are called linked genes
* Do not assort independently
* Dihybrid crosses deviate from expected 9:3:3:1 phenotypic ratio
Crossing Over- accounts for the recombination of linked genes.
 Recombination Frequency (RF) = # recombinants x 100
total # offspring
 Proposed by Morgan
 Process of crossing over during meiosis accounts for the recombination of linked genes (genes on
same chromosome)
 Crossing over—breakage and exchange of corresponding segments between homologous
chromosomes
--results in new allelic combination
 Probability of crossing over (recombination) between two genes is proportional to the distance
separating those genes
 The closer together two genes are, the less likely that a cross over will occur.
 Proved by A. H. Sturtevant
46. What are chromosome maps? What is their purpose?
Recombination frequencies are used to construct chromosome maps [show locations of genes on a
particular chromosome (linear)]
1 map unit = 1% recombination frequency (now called centimorgans)
47. What are the types of chromosomal mutations? Distinguish between each type.
A. Alterations of Chromosomal Number
B. Alterations of Chromosomal Structure- Breakage of a chromosome can lead to four types of
changes in chromosome structure
48. What is nondisjunction? What does it cause?
Nondisjunction
 In nondisjunction, pairs of homologous chromosomes do not separate normally
during meiosis
 Results in one gamete receiving two of the same type of chromosome (n+1) and the
other gamete receiving none (n-1)
49. Explain trisomy and monosomy, aneuploidy and polyploidy.
Aneuploidy
 Aneuploidy results from the fertilization of gametes in which nondisjunction occurred
 Offspring with this condition have an abnormal number of a particular chromosome
 a chromosomal aberration in which one or more chromosomes are present in extra copies or
deficient in number
 A monosomic zygote has only one copy of a particular chromosome
 A trisomic zygote has three copies of a particular chromosome
 Ex: Down’s Syndrome or Trisomy 21
Polyploidy
 Polyploidy is a condition in which an organism has more than two complete sets of chromosomes
◦ a chromosomal alteration in which the organism possesses more than two complete
chromosome sets.
 Triploidy (3n) is three sets of chromosomes
 Tetraploidy (4n) is four sets of chromosomes
 Polyploidy is common in plants, but not animals
 Polyploids are more normal in appearance than aneuploids
50. What disorders result from chromosomal mutations? Be able to distinguish between them.
Alterations of chromosome number and structure are associated with some serious disorders
Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals
surviving to birth and beyond
These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy
Examples of Autosomal Aneuploidy:
a. Down’s Syndrome
 Trisomy 21
 Related to age of parent
 Most common birth defect in U. S. (1/700 births)
b. Patau Syndrome
 Trisomy 13
c. Edward’s Syndrome
 Trisomy 18
Examples of Sex Chromosome Aneuploidy: (less severe)
a. Klinefelter’s Syndrome--Genotype usually XXY
b. Extra Y--XYY
c. Trisomy X (metafemales)--XXX
d. Turner’s Syndrome (monosomy X)--XO
Examples of Deletions:
a. Cri du chat Syndrome--Deletion on chromosome 5
Examples of Translocations:
a. Chronic Myelogenous Leukemia (CML)--Portion of chromosome 22 switched with a fragment
of chromosome 9
b. Type of Down’s Syndrome--Translocation from chromosome 21 to chromosome 15
51. What is genomic imprinting?
Genomic Imprinting
1. For a few mammalian traits, the phenotype depends on which parent passed along the alleles for
those traits
2. Such variation in phenotype is called genomic imprinting
3. Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint
during gamete production
4. “a variation in phenotype depending on whether an allele is inherited from the male or female
parent.”