Question

The \(A\) and \(B\) antigens in humans may be found in water-soluble form in secretions, including saliva, of some individuals \((\text {Se/Se and Se/se})\) but not in others (se/se). The population thus contains "secretors" and "nonsecretors." (a) Determine the proportion of various phenotypes (blood type and ability to secrete) in matings between individuals that are blood type \(A B\) and type \(O,\) both of whom are \(S e / s e\) (b) How will the results of such matings change if both parents are heterozygous for the gene controlling the synthesis of the H substance \((H h) ?\)

Short answer

Additionally, how would the results change if both parents are heterozygous for the H gene? In matings between AB and O blood type individuals who are Selse, the proportion of phenotypes in the offspring is as follows: 25% A secretor, 25% A non-secretor, 25% B secretor, and 25% B non-secretor. If both parents are heterozygous for the H gene, the results would change to: 37.5% A blood type, 37.5% B blood type, 18.75% O blood type, and 6.25% Oh blood type (Bombay phenotype).

Step-by-step solution

Determine the genotype of parents

For the first mating between blood type AB (IAIB) and blood type O (ii) parents, both of whom are Selse (they are secretors, so their genotype is Se/se). Parent 1: IAIB (AB blood type) and Se/se (secretor) Parent 2: ii (O blood type) and Se/se (secretor)

Set up Punnett squares for blood type and secretor genes

To determine the genotypes of offspring, we'll perform two separate Punnett square analyses: one for the blood type genes and one for the secretor genes. Punnett square for blood type genes: | IA | IB --------------- i | IAi| IBi --------------- i | IAi| IBi Punnett square for secretor genes: | Se | se -------------- Se | SeSe| Sese -------------- se | Sese| sesese

Determine the offspring's phenotype frequencies

Using the Punnett squares, we can determine the frequency of each phenotype among the offspring. Blood types: - IAi (A blood type): 50% - IBi (B blood type): 50% Secretor status: - SeSe (non-secretor): 25% - Sese (secretor): 50% - sesese (non-secretor): 25%

Calculate the proportion of various phenotypes in offspring

Using the frequencies obtained in step 3, we can calculate proportions of phenotypes in the offspring. Multiply the frequency of each blood type by the frequency of each secretor status to get the proportions: - A blood type (50%) and secretor (50%): 0.50*0.50 = 25% - A blood type (50%) and non-secretor (50%): 0.50*0.50 = 25% - B blood type (50%) and secretor (50%): 0.50*0.50 = 25% - B blood type (50%) and non-secretor (50%): 0.50*0.50 = 25% (a) Thus, in matings between blood type AB and blood type O individuals who are Selse, the proportion of various phenotypes in the offspring would be 25% A secretor, 25% A non-secretor, 25% B secretor, and 25% B non-secretor.

Analyzing the scenario with heterozygous H gene parents

In the second part of the exercise, we have to determine the results of matings between heterozygous H gene parents. Since both parents are heterozygous (Hh) for the gene controlling synthesis of the H substance, there will be a chance that the offspring will receive two recessive genes (hh) from both parents. A child born with the hh genotype will not be able to produce the H substance, which is needed for A, B, or O blood types. This lack of H substance leads to a rare blood type, called the Bombay blood type (Oh). To evaluate this possibility, perform a Punnett square analysis for the H gene: Punnett square for the H gene: | H | h ----------- H | HH | Hh ----------- h | Hh | hh Among these offspring, 25% would have the HH genotype, 50% would have Hh genotype, and 25% would have the hh genotype. We can also multiply the blood type frequencies with H substance frequencies: - AB blood type: 0 - A blood type: 0.50 * (0.25 + 0.50) = 0.375 - B blood type: 0.50 * (0.25 + 0.50) = 0.375 - O blood type: 0.25 * (0.25 + 0.50) = 0.1875 - Oh blood type (Bombay): 0.25 * 0.25 = 0.0625 (b) If both parents are heterozygous for the H gene, the results of such matings would change as follows: 37.5% A blood type, 37.5% B blood type, 18.75% O blood type, and 6.25% Oh blood type (Bombay phenotype).

Key concepts

Punnett square analysis

Punnett squares are a simple yet powerful tool in genetics that help predict the probability of different genotypes and phenotypes resulting from a particular genetic cross. Imagine them as a grid where you can organize potential allele combinations from the parents. In our example, we explore a mating between an individual with AB blood type (genotype \(IAIB\)) and one with O blood type (genotype \(ii\)). In a Punnett square, each parent's genotype is written along the top and left side of the grid.

• Parent 1: Alleles IA and IB
• Parent 2: Alleles i and i (since O type is recessive)

The offspring's possible genotypes are found by filling in the grid:

• IAi (A blood type)
• IBi (B blood type)

This gives us a 50% chance of each blood type, A and B. For secretor genes, using \(Se/se\) for both parents results in combinations: SeSe, Sese (secretor), and sese (non-secretor). These provide insight into phenotype frequencies such as being a secretor or non-secretor.

Secretor status

Secretor status in humans refers to the ability to secrete blood type antigens in bodily fluids such as saliva. This characteristic is determined by the Se gene, where the presence of at least one dominant allele (Se) grants secretor status.

• If your genotype is either SeSe or Sese, you are a secretor.
• If your genotype is sese, you are a non-secretor.

In our exercise, both parents have the genotype Se/se, which means they both can pass either the Se or se allele to their offspring. Using a Punnett square, we find the combinations:

• SeSe - 25% chance, secretor status confirmed
• Sese - 50% chance, secretor status
• sese - 25% chance, non-secretor

Secretor status affects the expression of the blood type antigen, thus sometimes impacting disease susceptibility and digestion processes. Knowing your secretor status is an interesting genetic insight into how your body's biochemistry works beyond just determining your blood type.

H substance synthesis

To understand the significance of H substance synthesis, we need to appreciate its role in blood type expression. The H gene facilitates the production of an essential precursor for A and B antigens called the H substance. Without it, the standard blood type antigens cannot be built.In the exercise, the parents are heterozygous for this gene, meaning they are \(Hh\) for H substance synthesis. A Punnett square analysis with these alleles produces:

• HH - 25% chance, normal H substance production
• Hh - 50% chance, still able to produce H substance
• hh - 25% chance, no H substance leading to Bombay blood type, Oh

The rare rr offspring can result in the Bombay phenotype, which lacks the H substance making a true O type. This occurs because without the H substance, neither A nor B antigens can be displayed, despite having their respective alleles. This phenomenon underscores the importance of epistatic interactions in genetics, where one gene can mask or alter the expression of another, leading to unexpected outcomes like the Bombay phenotype.