Mice normally have one yellow band on each hair, but...

Mice normally have one yellow band on each hair, but variants with two or three bands are known. A female mouse having one band was crossed with a male having three bands. (Neither animal was from a pure line.) The progeny were a. Provide a clear explanation of the inheritance of these phenotypes. b. In accord with your model, what would be the outcome of a cross between a three-banded daughter and a one-banded son?

Mice normally have one yellow band on each hair, but variants with two or three bands are known. A female
mouse having one band was crossed with a male having three bands. (Neither animal was from a pure line.) The progeny were
a. Provide a clear explanation of the inheritance of these phenotypes.
b. In accord with your model, what would be the outcome of a cross between a three-banded daughter and a one-banded son?

Key Concepts

Incomplete Dominance

Incomplete dominance is a pattern of inheritance in which the heterozygote expresses a phenotype that is intermediate between the two homozygotes. In the context of the mouse hair?banding problem, one allele produces the normal one band on each hair while the other allele produces the full mutant effect (three bands). Heterozygous mice, inheriting one copy of each allele, show an intermediate phenotype (two bands), thereby illustrating incomplete dominance.

Gene Dosage Effects

Gene dosage effects occur when the number of functional copies of a gene contributes additively to the phenotype. In this example, having two copies of the mutant allele results in the maximum effect (three bands), having one mutant allele produces an intermediate effect (two bands), and having no mutant alleles yields the normal condition (one band). This quantitative relationship between the number of mutant alleles and the number of bands is an example of a dosage effect.

Mendelian Segregation

Mendelian segregation is the principle that alleles separate during gamete formation and recombine at fertilization. This concept underlies the expected ratios in crosses; for instance, crossing a mouse with the homozygous recessive phenotype (one band, genotype NN) with a mouse displaying the homozygous dominant phenotype (three bands, genotype MM) results in heterozygous offspring (NM) with two bands. Further crosses, such as between a three?banded (MM) individual and a one?banded (NN) individual, produce a predictable outcome where all offspring are heterozygous (NM) with the intermediate phenotype (two bands).

Transcript

00:01 In mice, they bred or scientists bred a female mouse that has a one -banded marking around her fur with a male that has three bands.

00:17 And so that's our parental generation.

00:20 If we look at the f -1 ratios that were provided for us, there were two kinds of females.

00:26 Half of the females contained one band.

00:29 The other half of the females contained three bands.

00:37 Now, we can compare this to the males.

00:40 The males had one half of them with one band.

00:45 The other half had only two bands.

00:50 We see a phenotypic difference between females with one band and three bands compared to males that have a one band and two band pattern.

00:59 This phenotypic difference tells us.

01:02 That the banding gene must be located on the x chromosome.

01:15 And so that also tells us that males will express both alleles from the female in the parental generation.

01:30 So this mom here, and the question told us that they weren't true breeding, must contain an allele for one banded patterning and one allele.

01:42 For two banded patterning, because males are made, the sex of the offspring is determined by the male.

01:51 And so what that means is that there's a 50 % chance that this first set of offspring receive one allele from mom, and the other 50 % says that the other allele goes to the second half of the offspring.

02:07 And so if we allow, i will move this up just a little bit, if we organize this, let's say h1 equals one band, h2 equals two bands, and h3 equals three bands.

02:29 We can easily write the genotype for the female, the parent.

02:35 She has one band.

02:38 So that means she has at least an h1.

02:42 And her son has one band.

02:46 Now, you can look at the male, at the other male, and say, okay, he has two bands.

02:52 That means mom's genotype was h1, h2.

02:56 Because the male offspring will express both alleles from mom.

03:01 And so we know that the one banded female, who was the parent, has the genotype h1, h2.

03:09 Now, if we look at the male, we know that he's going to have a y chromosome in there, because that's what makes male flies male.

03:19 And he has to give this allele to the female to have three bands, because that banding pattern is not going to come from mom.

03:32 And so he must be h3y.

03:36 And so if we look at doing the same thing for the offspring, this first female who is one banded must be h1.

03:48 She has one and a hidden genetic diversity of h3 because she got one allele from mom and the other allele must have come from dad...

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