Suppose that the ability to taste PTC (phenylthiocarbamide)...

Suppose that the ability to taste PTC (phenylthiocarbamide) depends on the presence of a dominant gene $T$; nontasters are homozygous for the recessive gene $t$. This would be a case of classical dominant-recessive inheritance. (a) Consider a mating between heterozygous parents producing three offspring. What proportion of the offspring are likely to be tasters? What is the chance that all three offspring will be tasters? Nontasters? What is the chance that two will be tasters and one will be a nontaster? (b) Consider a mating between $T t$ and $t t$ parents. What is the anticipated percentage of tasters? Nontasters? What proportion can be expected to be homozygous recessive? Heterozygous? Homozygous dominant?

Key Concepts

Classical Mendelian Inheritance

This concept involves the transmission of traits from parents to offspring through dominant and recessive alleles. In the problem, tasting PTC is determined by a dominant allele (T), meaning only one copy of T is needed for the taster phenotype, whereas two copies of the recessive allele (t) result in the non-taster phenotype. The pattern of inheritance observed here is a cornerstone of classical Mendelian genetics, where segregation and independent assortment play key roles.

Punnett Squares

Punnett squares are a visual tool used to predict the genotypic and phenotypic outcomes of genetic crosses. They help in determining the probability of inheriting particular alleles from each parent. In this scenario, Punnett squares are used to trace the outcomes of crosses between heterozygous parents as well as between heterozygous and homozygous recessive individuals, outlining the distribution of tasters and non-tasters.

Genotype versus Phenotype

Understanding the difference between genotype and phenotype is crucial in this context. The genotype refers to the actual genetic makeup, such as TT, Tt, or tt, while the phenotype is the observable trait, which in this case is the ability or inability to taste PTC. This distinction is important because the dominant allele masks the presence of the recessive allele in heterozygous individuals, leading to the same phenotype in individuals with TT and Tt genotypes.

Probability Calculations in Genetic Crosses

Probability calculations are used to determine the likelihood of different genetic outcomes from a mating based on known genotypes. In this problem, the calculation involves determining the chances of producing various combinations of taster and non-taster phenotypes among the offspring, using principles of binomial probability for multiple offspring and using ratios derived from the Punnett square predictions.

Transcript

00:01 I'm going to look at today refers to the offspring containing the ability to taste phenothelicarbonide, which i'm going to abbreviate as ptc.

00:09 So as you can see, we're going to fill out out punnett square today.

00:14 So in order for an offspring to taste ptc, it has to have a dominant gene, which we will represent as capital t.

00:23 And people unable to taste ptc has to have a homozygous recessive gene, which we will represent as lowercase t.

00:33 So before before we get started, let's define homozygous recessive gene.

00:38 When you have a recessive allele and another recessive allele, that's when you have a homozygous recessive gene.

00:49 So remember that the prefix homo means the same, so they both contain the same allele.

00:58 And for the homozygous dominant gene, instead of being recessive, two recessive, it's two dominant alleles.

01:05 For the heterozygous dominant gene, or just heterozygous gene, you'll have one dominant gene or allele and one recessive allele.

01:17 So let's go to the first question.

01:20 If we have two heterozygous parents and three offspring, what will the proportion be of tasters and non -tasters? so if we have two heterozygous parents, we'll have a dominant allele and a recessive allele.

01:35 Since they're both heterozyas, i guess, be the exact same.

01:40 So in order to fill out pundit square, you simply fill it out by going over and just going down.

01:53 So dominant allele and a dominant allele.

01:56 And you'll do that for the next one as well.

02:02 Oops.

02:08 So then you'll go over to the next one and do a dominant allele, a recessive allele, and then you go to the next one.

02:24 I like writing my recessive genes or alleles to the right.

02:30 And the dominant ones to the left and do that and that's how you filled them out.

02:42 So now i want to make it look cleaner so i'll just fill it up quickly.

02:54 So again you just go down into the right and so now we have, now we're able to find our proportions.

03:07 So let's count how many dominant genes we have.

03:10 We have one, two, three.

03:13 So three out of four, because there's four, four squares.

03:17 One, two, three, four squares.

03:20 That gives us 75%.

03:25 And we have one recessive, one homozygous recessive, which is the inheritance of unable to taste ptc.

03:47 Over 4 equals 25%, and that is our proportion for this opponent square.

03:56 So we also want, so the next question is, what are the likelihoodness of all three offspring being tasters or non -tasters? so by this we're going to be using fractions.

04:11 So let me clear this right here.

04:21 So we have our two heterozygous parents still.

04:28 The question is still referring to the heterozygous parents...

Please give Ace some feedback