In humans, having attached earlobes (gene E) is a recessive trait

In humans, having attached earlobes (gene E) is a recessive trait. Those with the dominant trait have free earlobes. We will examine the inheritance of attached earlobes in Bob’s family. Bob’s mother has attached earlobes, but Bob’s father, Bob, and Bob’s wife do not. Bob has a son with attached earlobes and Bob’s wife is now pregnant with their second child. a) In the space to the right, please draw a pedigree showing the inheritance of attached earlobes in this family. b) Write each person’s genotype under their symbol. If a person’s second allele is unknown, please use a “-” as the second allele. c) What is the probability that Bob and his wife’s unborn child will not have attached earlobes? (Please fill in the Punnett square to show your work.)

The Correct Answer and Explanation is:

Answer:

a) Pedigree Diagram:

We will use the following symbols:

• Square = Male
• Circle = Female
• Shaded = Attached earlobes (recessive trait, genotype: ee)
• Unshaded = Free earlobes (dominant trait, genotype: EE or Ee)

rustCopyEdit       [Bob's Father] —— [Bob's Mother]
            ☐  (E-)     ○  (ee)
                   │
                 ┌─┴─┐
                ☐   [Bob's Wife]
              (Ee)   ○ (E-)
                 │
            ┌────┴────┐
           ☐          ?
         (ee)       (???)

b) Genotypes under each symbol:

1. Bob’s mother: ee (attached earlobes)
2. Bob’s father: E- (has free earlobes, genotype not fully known)
3. Bob: Ee (must have inherited one recessive e from his mother)
4. Bob’s wife: E- (free earlobes, but must be a carrier since their child is ee, so her genotype is Ee)
5. Their son: ee (attached earlobes, must have gotten one e from each parent)
6. Unborn child: Unknown

c) Probability and Punnett Square:

Since Bob is Ee and his wife is also Ee, we can fill in the Punnett square:

Resulting genotypes:

• EE → Free earlobes (25%)
• Ee → Free earlobes (50%)
• ee → Attached earlobes (25%)

Answer:
The probability that the unborn child will not have attached earlobes is:

• EE + Ee = 25% + 50% = 75%

Explanation

In humans, the inheritance of traits like earlobe attachment follows Mendelian genetics. Free earlobes (represented by the dominant allele E) will appear even if only one dominant allele is present. Attached earlobes are recessive and only show up if the individual inherits two recessive alleles (ee).

In this case, Bob’s mother has attached earlobes, meaning she must have the genotype ee. Since Bob does not have attached earlobes but his mother does, he must have inherited one e from her and one E from his father, making Bob heterozygous (Ee). Bob’s wife does not have attached earlobes either, but since she had a child with attached earlobes (ee), she must be a carrier, meaning her genotype is also Ee.

When two heterozygous individuals (Ee) have a child, the probability of each genotype can be determined by a Punnett square. The combinations result in:

• 25% chance for EE (free earlobes, not a carrier)
• 50% chance for Ee (free earlobes, carrier)
• 25% chance for ee (attached earlobes)

So, there is a 75% chance (3 out of 4) that their next child will have free earlobes and only a 25% chance the child will have attached earlobes. This follows basic rules of probability and inheritance in genetics.