AP Review
Chapters 13-15
Fast Facts
Heredity: the transition of traits from one
generation to the next
Along w/ similarities, there is also variationoffspring differ somewhat in appearance
from parents to offspring
Parents pass information to offspring through
coded hereditary units called genes.
- 30 to 40 thousand genes in humans
- genes are segments of DNA
Fast Facts
The human life cycle
- each somatic cell has 46
chromosomes (23 pair); 2 copies of
each pair are called homologous
chromosomes
- chromosomes can be displayed through
a karyotype
- pairs 1-22 are called autosomes
- pair 23 are called sex chromosomes
Fast Facts
Fast Facts
- gametes, or sex cells, contain 22
autosomes and 1 sex chromosome; a
cell with a single chromosome set is
called a haploid cell.
- haploid cells are abbreviated 1n (n =
23)
- the combining of gametes forms a
zygote; becomes a diploid cell (2n)
Fast Facts
Meiosis, like mitosis, is
preceded by the
replication of
chromosomes;
however, the single
replication is followed
by 2 consecutive
divisions (meiosis I
and meiosis II)
Fast Facts
Meiosis I
- during prophase I, chromosomes pair
up in synapsis; 4 chromatids form a
tetrad
- during metaphase I, homologous pairs
line up on equator
- during anaphase I, chromosomes, not
chromatids, separate to poles
Fast Facts
Meiosis II
- goes through the same steps as
meiosis I, but does not replicate DNA
Meiosis outcome is 4 1n gametes from a
single cell
Fast Facts
Crossing over
- when
chromosomes
line up along the
equator, parts of
chromatids can
combine genes
from parents
Sample Questions
Meiosis reduces chromosome number
and rearranges genetic information.
a) Explain how the reduction and
rearrangement are accomplished in
meiosis
b) several human disorders occur as a
result of defects in meiosis. Identify
one and explain how this could have
occurred.
Fast Facts
Punnett squares
are used to
predict the results
of a genetic cross
between
individuals of
known genotypes.
Fast Facts
Homozygous: when the alleles for a
gene are identical
Heterozygous: when the alleles for a
gene are different
Phenotype: physical appearance
Genotype: genetic makeup
Fast Facts
A testcross can be used to determine the
genotype of an organism that displays a
dominant trait.
- ex. Is the purple flower homozygous or
heterozygous?
Cross it with a recessive (white) flower.
The results will tell the genotype.
Fast Facts
A cross in which only one character is
discussed is a monohybrid cross.
If two characters are discussed, the cross is
a dihybrid cross
Incomplete dominance
- when the F1 hybrids have an appearance
somewhere in between the phenotypes of
the parents
Fast Facts
Codominance/Multiple Alleles
- the two alleles affect the phenotype in
separate distinguishable ways.
- ex. A, B, O blood types
Pleiotropy
- the ability of a gene to affect an organism in
many ways
- ex. the alleles for sickle-cell disease cause
multiple symptoms
Fast Facts
Epistasis
- a gene at one locus alters the phenotypic
expression of a gene at a second locus
Polygenic inheritance
- the additive effect of two or more genes on a
single character; not an “either or” situation
- converse of pleiotropy where a single gene
affects several phenotypic characters
Fast Facts
Pedigree: a family tree
that describes the
relationships
between generations
for a particular trait
- pedigrees are used
to map out specific
phenotypic traits
Fast Facts
Common recessive genetic disorders
- Cystic fibrosis: 1/2500 people of European
decent; 1/25(4%) are carriers
- Tay-Sachs disease:
- sickle-cell disease: 1/400 African-Americans
Common dominant genetic disorders
- Anchondroplasia: form of dwarfism;
- Huntington’s disease: no obvious phenotypic
effect until 35 to 45 yrs. old
Fast Facts
Chromosomal theory of inheritance: genes
have specific loci on chromosomes, and the
chromosomes undergo segregation and
independent assortment
Genes located on sex chromosomes are called
sex-linked genes
- Morgan’s evidence that a specific gene is
carried on the X chromosome helped confirm
the chromosomal theory of inheritance.
Fast Facts
linked genes: genes that are located on
the same chromosomes and tend to be
inherited together
- linked genes deviate from expected
Mendelian ratios
-ex. in flies body color and wing shape
are inherited together
Fast Facts
Fast Facts
Geneticists can use recombination data to
map a chromosomes genetic loci
Genetic map: an ordered list of the
genetic loci along a particular
chromosome
Linkage map: a genetic map based on
recombination frequencies
Fast Facts
Map of body-color
(b), wing-size (vg)
and cinnabar (cn)
- cn and b is 9%
- cn and vg is
9.5%
- b and vg is 17%
Fast Facts
Sex-linked disorders in humans
- muscular dystrophy: 1/3500 males in the
US
- hemophilia: absence of proteins for blood
clotting
Nondisjunction: the members of a pair of
homologous chromosomes do not move
apart properly during meiosis I or II.
- gametes are (n+1) or (n-1) if they have too
many or too few chromosomes
Fast Facts
Aneuploidy: having an abnormal
chromosome number
Trisomic: a chromosome in triplicate
(2n+1); ex. trisomy 21 (Down’s
syndrome)
Monosomic: if a chromosome is missing
(2n-1)
Fast Facts
Fast Facts
Polyploidy: organisms that have more
than two complete chromosome sets
- triploidy (3n)
- tetraploidy (4n)
Polyploids are more normal in
appearance than aneuploids
Fast Facts
Breakage of a chromosome can lead to
four types of changes in chromosome
structure
Deletion: occurs when a chromosomal
fragment lacking a centromere is lost
during cell division
Fast Facts
Duplication: a fragment becomes
attached as an extra segment to a sister
chromatid
Inversion: a chromosomal fragment can
reattach to the original chromosome in
reverse order
Translocation: the fragment joins a
nonhomologous chromosome
Fast Facts
Fast Facts
Alterations in chromosome number and
structure are associated with several
human disorders
- Down syndrome: 1/700 children born;
extra 21st chromosome
- Klinefelter syndrome: XXY male
- Turner syndrome: XO female
Fast Facts
Sample Question
In a laboratory population of diploid, sexually reproducing organisms a
certain trait is determined by a single autosomal gene and is expressed
as two phenotypes. A new population was created by crossing 51 purebreeding (homozygous) dominant individuals with 49 pure breeding
(homozygous) recessive individuals.
Generation Dominant Recessive Total
1
51
49
100
2
280
0
280
3
240
80
320
4
300
100
400
5
360
120
480
a) Identify an organism that might have been used to perform this experiment,
and explain why this organism is a good choice for conducting this
experiment.
b) On the basis of the data, propose a hypothesis that explains the change in
the phenotype frequency between generation 1 and generation 3.
In a laboratory population of diploid, sexually reproducing
organisms a certain trait is determined by a single autosomal
gene and is expressed as two phenotypes. A new population was
created by crossing 51 pure-breeding (homozygous) dominant
individuals with 49 pure breeding (homozygous) recessive
individuals.
Generation Dominant Recessive Total
1
51
49
100
2
280 0
280
3
240 80
320
4
300 100 400
5
360 120 480
a) Identify an organism that might have been used to perform
this experiment, and explain why this organism is a good
choice for conducting this experiment.
b) On the basis of the data, propose a hypothesis that
explains the change in the phenotype frequency between
generation 1 and generation 3.