Lecture 17 Mendelian Genetics Flashcards
Genetics Terminology
P1 generation
– parental generation
F1 generation
first offspring from the parental generation
– F2, F3 etc
Alleles
different forms of a gene
Homozygous and Heterozygous
Homozygous
- having two of the same allele for a given gene
Heterozygous
– having two different alleles for a given gene
Genotype
– the combination of alleles in an individual
Phenotype
– the expression of a trait in an individual
Dominant
– the trait that is phenotypically expressed in a heterozygous individual
Recessive
the trait that is phenotypically expressed when
two copies of the same allele are present
will be masked by a dominant gene
True-Breeding
True breeding plants are _____ and each produces one type of gamete
one parent will only make gametes with the A allele, when the other parent will make gametes with the a allele
all of the offspring will be heterozygous Aa for the gene and show?
One parent will only make gametes
with the A allele, whereas the other
parent will make gametes with the a
allele.
* All of the offspring (F1 generation) will
be heterozygous, Aa, for the gene
and show the dominant phenotype.
*True breeding plants are homozygous and each produces one type of gamete
AA X aa (p1 generation)= Aa (F1 generation)
Segregation: The Monohybrid Cross
True-breeding parents (P1 generation)
True-breeding parents (P1 generation)
* All of the offspring (F1 generation) will be
heterozygous, Aa, for the gene and show the
dominant phenotype.
* Cross of F1 progeny
* monohybrids i.e. heterozygous for
one character
* Resulting phenotypic ratio is 3:1
* Resulting genotypic ratio is 1:2:1
* Genotypic ratios can be different
from phenotypic
ratios
* Note: genes come in pairs that segregate
in the formation of gametes–> segregation
In certain plants, tall is dominant to short. If a
heterozygous plant is crossed with a homozygous
tall plant, what is the probability that the offspring
will be short?
A) 1
B) 1/2
C) 1/4
D) 1/6
E) 0
0
The Testcross
What if you were given a yellow seed and asked to determine the
genotype?
* Yellow is dominant, so genotype can be AA or Aa
* Perform a testcross:
– Cross the yellow seed with a green seed and observe the
phenotype of the seeds resulting from the cross
Why use a green seed
in the cross
Segregation
segregations reflects the separation of homologous chromosomes during anaphase I of meiosis
*copies of A allele in replicated chromosome
*copies of a allele in replicated homologous chromosomes
In humans, being a tongue roller (R) is dominant over non-
roller (r). A man who is a non-roller marries a woman who is
heterozygous for tongue rolling. What is the probability of this couple having a child who is a tongue roller?
Practice Problem
A pet angelfish breeder is looking to breed her champion
black lace angelfish. She knows that black colour is
dominant to white but doesn’t know if her angelfish is
purebred. How can she find out?
Degrees of Dominance
Incomplete Dominance
Incomplete dominance can occur because neither of the two alleles is fully dominant over the other, or because the dominant allele does not fully dominate the recessive allele
- Incomplete dominance
– The phenotype of F1 hybrids is
somewhere between the
phenotypes of the two parental
varieties
– Neither red or white allele is
completely dominant
– Flowers of the heterozygous have
less red pigment than red
homozygous dominants - The original traits show up again
when you cross the F1’s
Degrees of Dominance
Mendel’s classic pea experiments:
– The F1 offspring always looked like one of the two
parental varieties
– Due to one allele showing complete dominance over
the other
– But, many genes do not follow Mendelian inheritance
* Degrees of Dominance
– Alleles can show different degrees of dominance and
recessiveness with relation to each other
Mendels law of dominance simplified
in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively.
Practice Problem
* An Angora rabbit (long fur) is crossed with a short-
furred Rex rabbit. All of the offspring have medium
length fur. What are the genotypes of the parent
rabbits?
* What is the mode of inheritance?
* What could you do to confirm this?
Independent Assortment
The principle of independent assortment
states that segregation of one set of alleles of
a gene pair is independent of the segregation
of another set of alleles of a different gene
pair.
* The hereditary transmission of either gene has
no effect on the hereditary transmission of the
other
Determining Gametes
What gametes are possible for each parent in the
following crosses? (assume independent assortment):
AaBB x Aabb
AABBCC x aabbcc
n guinea pigs, the brown coat colour allele (B) is dominant
over red (b) and the solid colour allele (S) is dominant over
spotted (s). The F1 offspring of a cross between true-
breeding brown, solid-coloured guinea pigs and red, spotted
guinea pigs are crossed. What proportion of their offspring
(F2) would be expected to be red and solid coloured?
A. 1/9
B. 1/16
C. 3/16
D. 9/16
E. 3/4
Genes That Modify Phenotype and phenotypic ratios
Epistasis
Interaction of genes can affect the same trait
– How?
* The products of two or more genes result in a certain
phenotype; or
* The product of one gene masks or changes the expected
phenotype of one or more other genes
* See modification of expected ratio!
– Textbook example: Chickens (Figure 14.25 p.312)
The F2 generation of a cross between white leghorn and white Wyandotte chickens displays the modified ratios 13:3 both breeds are white but for different genetic reasons, there are two genes involved in pigment production, each with two alleles, the C gene encodes a protein that affects coloration in feathers, the domiantn allle C produces pigment and the recessive allele c does not produce pigment. A different gene, I codes for the inhibitor protein, the product of the dominant allel I, inhibits the expression of C, whereas the recessive allele, i does not produce the inhibitor and so does not inhibit the expression of C, thus the white leghorn genotype CC II) is white because the product of the dominant allele I inhibits the pigment in the feathers and the white Wyandotte (genotype cc ii) is white because the recessive allele c does not produce feather pigment to begin with. The F1 generation has a genotype Cc Ii and is also white, with independent assortment only the three C-ii offspring have colored feathers in the F2 generation, the rest have white feathers so the ratio of white: colored is 13;3
*Product of C gene(genotypes CC and Cc)–>pigment
Inhibitor of C gene (II and Ii)
*White leghorn is white because the inhibitor allele I blocks expression of the pigment allele C
*White Wyandotte is white because the pigment allele c does not produce pigment
* Genotypes CC ii and Cc and ii have colored feathers wherease all other genotypes have white feathers, the result is and F2 rarios of white:colored of 13:3 which is modified formed of the expected 9:3:3:1
epistasis is the interaction of genes affecting the same trait and it can modify the 9:3:3:1 ratio of phenotypes in this example to 13:3
* Must know this example!
– Colouration in Labrador Retrievers is an example of
epistasis
Epistasis in Labrador Retrievers
epistasis in Labrador Retrievers
– In this example, a gene at one locus alters the phenotypic expression of a
gene at a second locus.
– One gene determines whether the hair pigment is black (B) or brown (b).
* The black allele (B) is dominant to the brown allele (b).
– Another gene, the epistatic gene, determines whether the dogs will have
pigment deposited in hair (E) or not (e)
* The pigment allele (E) is dominant to absence of (e) pigment.
– An (ee) individual has no pigment (i.e. yellow) regardless of the genotype
of the first gene.
* BBee, Bbee, bbee all yellow labs
Gene S controls the sharpness of spines in a type of cactus.
Cactuses with the dominant allele, S, have sharp spines, whereas
homozygous recessive ss cactuses have dull spines. At the same
time, a second gene, N, determines whether or not cactuses have
spines. Homozygous recessive nn cactuses have no spines at all.
If doubly heterozygous SsNn cactuses were allowed to self-
pollinate, the offspring would segregate in which of the following
ratios?
A) 3 sharp-spined:1 spineless
B) 1 sharp-spined:2 dull-spined:1 spineless
C) 1 sharp-spined:1 dull-spined:1 spineless
D) 1 sharp-spined:1 dull-spined
E) 9 sharp-spined:3 dull-spined:4 spineless
Lecture 17 reading
Segregation of alleles takes place in human meiosis
dominant traits appear in every generation, brachydactyly (the middle long bone in finger fails to grow: the finger remains short results from a mutation in a gene whose nonmutant product is a protein involved in cartilage formation, which is needed for bone growth)
Features of the pedigree that suggest dominant inheritance
affected individuals are equally likely to be females or males
most mating’s that produce affected offspring have only one affected parent, the brachydactyly trait is rare, so a mating between two affected individuals is extremely unlikely
among mating in which one parent is affected approx. half the offspring are affected
Recessive traits skip generations
recessive inheritance shows a pedigree pattern diff from dominant inheritance,
it is not unusual for inherited traits to have elevated frequencies among particular subpopulation, this type of albinism is due to a mutation in the gene OCA2. which encodes a membrane transporter protein thought to be important in transport of the amino acid tyrosine which is used in synthesis of the melanin pigment that is responsible for skin, hair and eye color
principal pedigree characteristics of recessive traits
the trait make skip one or more generations
affected individuals are equally likely to be females or males
affected individuals may have unaffected parents, as in the offspring of the second mating in the second generation for a recessive trait that is sufficiently rare almost all affected individuals have unaffected parents
affected individuals often result from mating between relatives, typically first cousins
Many genes have multiple alleles
