Question

  In the notation A = normal, a = albino, the genotype AA describes

   A) heterozygous B) homozygous dominant C) homozygous recessive

Answer 1

In the notation A= normal, a= albino, the genotype AA describes

Answer is B.

Homozygous dominant

Explantion:----

Homozygous is a word that refers to a particular gene that has identical alleles on both homologous chromosomes. It is referred to by two capital letters (XX) for a dominant trait, and two lowercase letters (xx) for a recessive trait.

A number of genes come in two variants: dominant and recessive alleles. In diploid life forms – like humans, mice, and many plants – that contain two copies of each gene, the combination of genes present determines whether that organism is heterozygous or homozygous in the expression of that given gene feature. Having one dominant allele and recessive allele each makes the organism heterozygous, but having two of a single allele makes the organism homozygous. If an organism has two copies of the same dominant allele, that organism is known as homozygous dominant – making the expression of that gene certain in the organism, and incredibly likely in its offspring.

• The dominant trait for eye color is brown, represented by BB. All other eye colors – blue, grey, green, and hazel – are recessivetraits, represented by bb. A homozygous brown eyed person would have the BB gene, while a homozygous blue eyed person would have the bb gene.

• An individual has the dominant SS gene, a homozygous gene that results in farsightedness. However, his friend has the homozygous gene ss for the recessive trait, which is normal sight.

• One girl’s blood clots normally, so her homozygous gene is the dominant CC. A boy in her class has hemophilia, which is present in the recessive gene cc.

• Having dimples on the cheeks is a dominant trait, with the homozygous DD gene causing them. However, the recessive trait is distinguished by the symbol dd; individuals with this homozygous gene will not have freckles.

• Curly hair is a dominant trait, with the homozygous HH gene causing the condition. Its recessive trait, represented by the homozygous gene hh, is straight hair.

Homozygous Dominant Example:---

Homozygous Dominant Dimples

Let's talk about homozygous dominant genotypes and Luke. Luke's parents are Sally and John. Both Sally and John have dimples, which is a dominant trait. Furthermore, let's assume that both Sally and John have two alleles for this dominant gene, represented as "DD." In this case both Sally and John have a homozygous dominant genotype. Because Luke gets one allele from Sally and one from John, Luke has a 100 percent chance of inheriting a homozygous dominant genotype of "DD," and Luke will be born with dimples.

Dominant and Recessive Alleles

The alleles an offspring inherits can be either dominant or recessive. When an allele is dominant, it means the genetic characteristic it is defining is more likely to occur. For example, having an immunity to poison ivy is a dominant trait in humans. When a gene is recessive, it means it is less likely to occur. For example, color blindness is a recessive trait.

Homozygous Curly Hair

Curly hair is another dominant trait. In this example, assume that both Sally and John have curly hair, but that Sally has a heterozygous genotype of "Cc." Since Luke gets one allele from Sally and one from John, Luke has a 50 percent chance of having a homozygous dominant genotype of "FF" and a 50 percent change of having a heterozygous dominant genotype of "Ff." In this example, Luke will physically display curly, but he will carry a recessive allele for straight hair.

Homozygous Freckles

Having freckles is a third example of a dominant trait. In this example, assume that both Sally and John have freckles . Let's also assume that both Sally and John have one dominant allele for freckles and one recessive allele. Sally and John would both have a heterozygous genotype for freckles, which would be represented as "Ff." Does that mean Luke will too? Not necessarily. Because Luke gets one allele from Sally and one from John, Luke has a 25 percent change of have a homozygous dominant genotype of "FF," a heterozygous dominant genotype of "Ff" and a homozygous recessive genotype of "ff."

Explanation:

In a monohybrid (single trait) cross, there are three possible genotypes.

The Genotype is the possible pair of traits from the parents represented by letters called alleles

The Phenotype is the possible trait displayed by the genotype.

Using the Alleles
T = Tall
t = short

The three possible genotypes (pairs of alleles) are:

TT = Homozygous Dominant for Tall

Tt = Heterozygous Dominant for Tall

tt= Homozygous Recessive for Short

If we use the allele symbols A(normal) and a(albino), the man’s genotype must be Aasince he is normal but one of his parents is aa. The woman has an albino brother, which means both her parents must be carriers (Aa). However, the woman (who is not albino) could have either an AAor Aa genotype.

In the woman’s case, the aa (albino) genotype must be excluded as a possibility. Therefore, the probability of the woman being AAis 1/3, and the probability that she is Aais 2/3. First, we should calculate the probability of the couple having an albino child each time a child is born. If the woman is Aa, then the mating is Aax Aa, and P(albino) = 1/4. However, there is only a 2/3 chance that she is Aa. So overall for this mating, P(albino) = P(man is Aa) x P(woman is Aa) x 1/4 = 1 x 2/3 x 1/4 = 2/12 = 1/6. Conversely, the probability that a child will be normal P(normal) = 1 – P(albino) = 5/6.If the couple plans to have 2 children, there are 4 possible outcomes, which are given in the table below along with each probability.

The overall probability of each outcome is calculated using the multiplication rule.Child 1Child 2ProbabilityNormal (5/6)Normal (5/6)25/36Normal (5/6)Albino (1/6)5/36Albino (1/6)Normal (5/6)5/36Albino (1/6)Albino (1/6)1/36a.The probability that at least one child will be albino corresponds to the last three outcomes of the table. Since it can happen in any of three different ways, the three probabilities should be added to get the final probability. P(at least one albino) = 11/36 = 0.306.b.

The probability of both children being normal is given in the table. P(both are normal) = 25/36 = 0.694.38.

Albinism is recessive to normal body pigmentation in man. It is an autosomal trait. If a homozygous normal man marries an albino girl, what would be the phenotypic and genotypic ratios in F2 generation from this marriage?

Since the two traits (albinism & normal body colour) are present in autosomes, they would behave as Mendelian monohybrid cross. Since albinism is recessive it might be denoted with “cc” alleles and normal body pigmentation by “CC” genes because both are homozygous.

solution:

The marriage would produce,

a)

phenotypically 3 normal children and 1 albino child.

(b) The marriage would produce, genotypically, 1 normal homozygous; 2 normal heterozygous; and 1 albino homozygous child

The F2 ratio of 1 true-breeding round : 2 hybrid round : 1 true-breeding wrinkled is consistent with each individual having 2 copies of the gene for seed shape. The two gene copies can be the same or different. Different versions of a gene are called alleles. The combination of alleles carried by an individual is its genotype. For example, let’s say R (read “big R”) is the dominant round allele, and r (read “little r”) is the recessive wrinkled allele. True-breeding round peas would have two copies of the R allele and a genotype of RR. True-breeding wrinkled peas would have two copies of the r allele and a genotype of rr. Hybrid round peas would have one copy of the R allele and one copy of the r allele, and a genotype of Rr.

P			RR x rr
F1			Rr
F2	1 RR	:	2 Rr	:	1 rr
F3	RR		3 R- : 1 rr		rr

An individual who carries two copies of the same allele (i.e. RR or rr) is called a homozygote (homo = same, zygote = embryo). Homozygotes produce genoctypially identical offspring when allowed to self-fertilize. There are dominant homozygotes (i.e. RR individuals) and recessive homozygotes (i.e. rr individuals). An individual who carries two different alleles (i.e. Rr) is a heterozygote (hetero = different).

A dash (-) is used to represent an unknown allele or a mix of alleles. For example, a round pea randomly picked from Mendel’s garden would have the genotype R-, since it could be either homozygous dominant or heterozygous. The ratio of 3 round : 1 wrinkled could be depicted as 3 R- : 1 rr because the round peas include both heterozygotes and homozygotes.

The dominant allele is often assigned a capital letter, and the recessive allele is assigned the corresponding lowercase letter. The letter chosen is often the first letter of the dominant trait. For example, Y might be the allele for yellow seeds, while y is the allele for green seeds. P might be the allele for purple flowers, while p is the allele for white flowers. Also notice that the alleles are italicized (or underlined when written by hand). If you go on to study more advanced genetics, you will learn more complicated systems of notation. After all, there are only 26 letters in the English alphabet and thousands of traits! For the time being, it’s good to practice using this basic system of notation.