Genetics
Learning Target Objectives: (I can …)
• Analyze pedigrees, determining who has, carries, or is free of a trait.
• Create Punnett squares for monohybrid and dihybrid crosses
• Predict the genotype, phenotype and the ratios of each in the offspring of a
homozygous, heterozygous, etc. cross.
• Compare and contrast inheritance for sex-linked traits, codominance,
incomplete dominance, simple dominant and recessive traits, and multiple
allele situations.
• Explain the significance of Gregor Mendel’s work and the use of a testcross.
• Apply Mendel’s 3 principles
• List common genetic disorders or diseases
Vocabulary:
Gregor Mendel * heredity * pure * strain * self-pollination *
pollination * cross-pollination * P1 * F1 * F2 * gene * allele
* dominant * recessive * genotype * heterozygous *
homozygous * phenotype * multiple alleles * monohybrid cross
* Punnett square * probability * gametes * testcross *
codominance * incomplete dominance * dihybrid cross * germ
cells * somatic cells * mutation * deletion * inversion *
translocation * nondisjunction * monosomy * trisomy * point
mutation * frameshift mutation * mutagens * autosomes *
sex chromosomes * sex-linked trait * linkage group *
population sampling * pedigree * carriers * codominant
Genetics
Gregor Mendel – statistically determined the probability for the appearance of
certain traits. (Ex: What are the chances of a yellow bean being produced
when a green bean and a yellow bean are crossed?)
Heredity – transmission of traits from parents to offspring
Pure – offspring will always have that trait (Ex: always have red flowers)
Strain – All plants in family lineage are “pure” for that specific trait
Self-pollination - fertilization occurs between flowers on the same plant (Or it
may be between the male/female parts of the same flower)
Pollination – pollen transfer from anther to stigma
P1 – (parental generation) a strain used for pollination/genetic tests
P1
x
(green pod)
P1
(yellow pod)
Cross-pollination
Must be 2 different plants since
these are pure for their trait
F1
and 1 is green and 1 is yellow
F1 (first filial generation) – offspring of the P1 generation (a cross of 2 different
pure strains)
F2 (second filial generation) - is produced by self-pollinating the F1 generation
F1
x
F2
F1
Self-pollination
Gene – segment of a chromosome that has nucleotide codes for a certain trait
Allele – the contrasting, or partner gene, for a given trait. There is 1 allelle on
each homologous chromosome for a total of 2 alleles per trait. (Assuming only
1 set of alleles controls that trait.)
Alleles are labeled to indicate dominance. For example, since green pods are
dominant over yellow, the alleles for pod color are labeled:
G (to show the dominant green allele is present) A capital letter is
always used to show the dominant allele.
or
g (to show the recessive yellow allele) A lower case letter (of the
same letter of the alphabet) is always used for the recessive
allele.
A pure green plant would have both alleles for green. It is: GG
A pure yellow plant would have both alleles for yellow. It is: gg
Mendel found the F1 generation often only expressed (showed) one trait. Ex:
crossing peas of pure green pods to peas with pure yellow pods produced peas
with all green pod offspring. So he came up with “The Principal of Dominance
and Recessiveness.”
Dominant allele – a gene that masks its partner gene on the homologous
chromosome. Ex: If “Dad” gives a smooth pea gene (allele) and “Mom” gives a
wrinkled pea gene (allele)but the offspring are all smooth, we know smooth is
dominant because it is expressed.
Recessive allele – these alleles are masked by dominant alleles. They are NOT
expressed traits unless BOTH alleles are for the recessive trait. So, if only 1 of
the 2 alleles is dominant, the offspring only show the dominant trait.
Mendel’s 3 Principles:
1)
Principle of Dominance and Recessiveness: 1 factor (dominant) may mask
another (recessive) preventing it from being seen.
2)
Principle of Segregation: “Factors” (genes) separate during egg or sperm
production so offspring will have a total of 2, not 4, factors for that trait.
3)
Principle of Independent Assortment: Factors (genes) for different traits are
not connected. In other words, a plant dominant for green pods could show
the recessive trait for height.
Genotype – the actual genetic makeup of an organism. This can be shown using
trait alleles. Exs:
GG - means both of the plant’s alleles are for the dominant green pod
Gg - shows that the pea pod is green but the plant has a recessive gene for
yellow pod color.
gg - shows the pod will be yellow because both alleles code for the recessive
yellow color.
Homozygous traits - BOTH alleles for the trait are the same. Ex: GG or gg
Heterozygous traits - means the alleles for that trait are different Ex: Gg
Phenotype – the appearance of an organism (The expressed trait)
Ex: GG and Gg are 2 different genotypes but since both code for green pods,
they have the same phenotype.
------------------Multiple alleles – 3 or more alleles (in the general population) control a trait.
Blood can be A, B, AB, or O type blood. Alleles for blood include IA, IB, and i.
But still only 2 alleles can be found in any ONE individual. The other alleles,
however, are found in the population at large. (Remember we get only 1 allele
from each parent for a total of 2.)
Remember: A difference in phenotype (appearance) indicates a difference in
genotype BUT this is not necessarily true in reverse.
Ex: Gg and gg different phenotype and different genotype
GG and Gg different genotype but SAME phenotype
However, phenotype is determined by genotype (Ex: You can’t get yellow from
Gg or GG, you have to have a gg genotype)
Monohybrid cross – breeding together organisms that differ only in one trait
(That is, only one trait is being considered).
Punnett square - shows the possible genotypes that might result from a cross.
Ex: Punnett square for a monohybrid cross looking at pod color
g
g
G
Gg
Gg
G
Gg
Gg
Shows 1 parent’s genotype
Boxes show possible genotypes of offspring
Shows the other parent’s genotype
In this example, one parent is pure for green (homozygous dominant) and one
parent is pure for yellow (homozygous recessive). The offspring are all green
heterozygotes (carry the yellow gene)
Punnett squares can be used to show probability (how likely the offspring are
to inherit a certain genotype or how likely they are to exhibit a certain
phenotype). It shows possible genotype combinations in the offspring.
Dad’s alleles
G
g
G g
GG Gg
Gg gg
Mom’s alleles
Shows the only 4 unions of gametes that
can occur.
The genotypes show a 2 out of 4 chance (likelihood) of carrying the recessive
“g” allele. (50% chance)
The phenotype = a 1 out of 4 chance (25%) of being yellow, 3 out of 4 chance
(75%) of being green.
Gametes – the sex cells (egg and sperm) Because we get only one set of
chromosomes from each parent (meiosis), if they have an allele for a green
pod on one chromosome and an allele for yellow on the other homologue
(chromosome partner), we don’t know which allele the offspring might get
(50% chance of getting either one from that parent).
Testcross – breeding a phenotypically dominant individual to a homozygous
recessive individual to determine the genotype of the phenotypically
dominant individual.
(We know any individual showing the recessive trait is homozygous recessive
for that trait. However, a phenotypically dominant individual could be
homozygous dominant or heterozygous.)
Exs:
G
g Gg
g Gg
? G
Gg
Gg
Phenotypically, all offspring
are green If parent is
homozygous dominant (GG).
G
g Gg
g Gg
? g
gg
gg
Phenotypically, some will
have yellow pods (50%),
If parent is heterozygous
(Gg).
Single gene cross possibilities:
Homozygous dominant x homozygous recessive:
G
G
g Gg Gg
g Gg Gg
Offspring are 100% green phenotype.
Genotypically, they are 100% Gg (all
“carry” the yellow gene – heterozygous)
Homozygous x heterozygous crosses:
G
G GG
g Gg
G
GG
Gg
Phenotype: 100% green
Genotype: 50% homozygous dom.(green)
50% heterozygous (Gg)
g
g
G Gg Gg
g gg gg
Phenotype: 50% green, 50% yellow
Genotype: 50% homo.recessive (yellow)
50% heterozygous (Gg)
Heterozygous x heterozygous cross:
G
g
Phenotype: 75% green & 25% yellow (3:1 ratio)
G GG Gg
Genotype: 25% homozygous dominant, 50% heterozygous,
g Gg gg
25% homozygous recessive (1:2:1 ratio)
Codominance and Incomplete dominance
BOTH alleles influence the trait.
With codominance, if a red flower allele were united with a white flower
allele, the offspring would have red and white striped flowers or some other
combination of red and white.
With incomplete dominance, if a red flower allele were united with a white
flower allele, the offspring would have pink flowers.
Usually, with codominant traits, one uses the capital letter and the other uses
the same capital letter but with a prime sign.
Ex: Red = R and White = R’
RR’ = red and white (or pink in incomplete
dominance)
Dihybrid cross - looks at 2 pair of alleles and the chance of getting both
dominant traits, both recessive traits, etc. This yields 16 possible gene
combinations in a heterozygous x heterozygous cross for both traits. From
these 16 genotypes, there are only 4 phenotypes . There will be:
9 phenotypically dominant in both traits
3 phenotypically dominant in one trait
3 phenotypically dominant in the other trait
1 recessive in both traits
***That’s a 9:3:3:1 ratio.
Ex: AaBb x AaBb parents can produce eggs or sperm with
these combinations: AB, aB, Ab, ab which we can plug into a Punnett square.
AB
aB
Ab
ab
AB
AABB
AaBB
AABb
AaBb
aB
AaBB
aaBB
AaBb
aaBb
Ab
AABb
AaBb
AAbb
Aabb
ab
AaBb
aaBb
Aabb
aabb
Germ cells = sex cells (eggs or sperm --> gametes) (1N) (formed via meiosis)
Somatic cells = body cells (2N) (divide by mitosis)
Mutation – a change in DNA (usually a change in base sequencing which can
change which proteins are or are NOT made). When germ cells are affected,
the offspring may show a deformity.
Chromosome mutations:
Deletion – a piece of chromosome breaks off so all of that information is lost
Inversion – a piece of chromosome breaks off, flips around and reattaches
upside down in place.
Translocation – a chromosome piece breaks off and reattaches to a different,
nonhomologous chromosome.
Nondisjunction – during meiosis, either homologous chromosomes or sister
chromatids fail to separate (anaphase I or anaphase II) so 1 daughter cell gets
an extra chromosome and 1 daughter cell is missing a chromosome. (This may
also happen during mitosis but usually is not as serious.)
Abnormal Anaphase I:
Abnormal Anaphase II:
XX
X
nothing
XX
l l
Anaphase ll:
nothing
nothing
gametes
nothing
ll
ll
should only be 1 chromatid in these
gametes
Nondisjunction causes:
Monosomy – missing 1 chromosome out of the entire set (Ex: 45 instead of 46)
Trisomy – has 1 extra chromosome Ex: 47 instead of 46 chromosomes
Down’s Syndrome – retardation & swollen face due to trisomy 21.
Klinefelter’s syndrome – extra “X” chromosome (trisomy 23), low fertility, less
male chest hair, etc., may be learning disabled. (XXY) This is a male condition.
Turner’s Syndrome – (monosomy 23) female; will be underdeveloped and
sterile (X “0”)
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Go to
Section:
Meiosis II
Gene mutations:
Point mutations – change in 1 base (A, T, C, G) on a gene. It may be a
substitution of the wrong base, an addition of a base, or the loss of a base.
This can result in the wrong amino acid(s)/protein being formed.
Frameshift mutation – a point mutation where a base is added or deleted. This
results in all of the following bases read out of order.
(*Don’t need to draw this.)
Ex: ACT TCA GCT CCG 3 bases are read together to code for an a. acid
AGC TTC AGC TCC G… Inserting 1 base changes EVERY codon for
every amino acid from that point on.
Mutagens – anything that damages DNA (Ex: sunlight, radiation, cigarettes)
Sex Chromosomes = X and Y (chromosome #23) In humans, females have
XX and males have an XY combination. Since only males carry the Y
chromosome, they control the child’s sex.
Dad’s
Genes
X
Y
X
XX
XY
X
XX
XY
Mom’s genes
Daughters
Sons
Autosomes – all chromosomes NOT involved in sex determination (#1 – 22)
Sex-linked trait : genes for a trait may be carried on only one type of sex
chromosome (Usually the X chromosome) and therefore may only appear in
one sex (males) most of the time (Ex: color blindness & hemophilia).
Moms act as carriers (or have subclinical symptoms).
Linkage group – group of genes found on one chromosome. These USUALLY
travel together & are inherited as a group. (Except sometimes with crossingover). But, the closer 2 genes are on a chromosome, the less likely they’ll be
affected by crossing over.
Studying human traits may involve population sampling. Here a small
cross-section of the population is used to represent the entire population. (Ex:
100 people are randomly selected and 40% have hairy knuckles, then it might
be assumed that 40% of the entire population has hairy knuckles.)
Pedigree – record of several generations showing how traits were inherited.
By analyzing a pedigree, we can sometimes find carriers (Someone who has 1
recessive gene but does not show it in the phenotype – heterozygous).
For a recessive trait, we use these symbols:
Males:
Females:
Homozygous dominant
(non-carrier)
Heterozygous (carrier)
Homozygous recessive
(displays recessive trait)
Figure 14-3 A Pedigree
Section
A horizontal
line14-1
connecting a male and
female represents a
marriage.
A half-shaded circle
or square indicates
that a person is a
carrier of the trait.
A completely
shaded circle or
square indicates
that a person
expresses the
trait.
Go to
Section:
A circle
represents a
female.
A square
represents a
male.
A vertical line and
a bracket connect
the parents to
their children.
A circle or
square that is
not shaded
indicates that
a person
neither
expresses the
trait nor is a
carrier of the
trait.
Single Allele Traits – only 1 allele controls the trait.
Ex: Sickle Cell Anemia – A = normal gene, A’ = sickle shape gene
These are codominant (both are expressed if present):
AA = all normal red blood cells
AA’ = ½ normal and ½ sickle cells
A’A’ = Sickle Cell Disease (all cells are sickle shaped)
AA’ (similar to carriers) – have resistance to malaria and mild or no sickle cell
disease symptoms.
Ex: Huntington’s Disease – here a dominant gene exists for the
disease but doesn’t appear until adulthood.
Polygenic Traits:
Traits controlled by more than 1 set of genes. (Ex: skin and eye color – how
dark the color will be.)
Multiple Allele Traits:
One set of genes controls the trait but there are 3 or more alleles possible (in
the population’s gene pool). Ex: Blood types: IA, IB, iO
Sex-influenced Traits:
The trait, in heterozygotes, is only expressed in males, for example.
Ex: Baldness:
BB - both men and women go bald
BB’ - Only men go bald
B’B’ - neither goes bald
(Hormones affect the trait.)
Ways to Detect Genetic Disorders:
Karyotype – a picture of each chromosome and its homologue. These are
looked at for abnormalities.
Amniocentesis – fluid is removed from the embryonic sac. The chromosomes
from cells are then karyotyped.