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ScienceNet - Life Sciences - Genetics/ Reproduction
 

Question No.  19243 :
I understand that traits such dimples and unattached earlobes are considered dominant traits. Is it possible for a child to have such traits when both parents do not exhibit such traits? Is it possible for such traits to "skip a generation"?

Visible human characteristics or traits are controlled by groups of information called genes that are composed of nucleotides. Each gene codes for a particular protein. (What actually determine our traits are proteins, not DNA.) Many genes are combined to form a large piece of DNA called a chromosome. Each normal human being has 46 chromosomes, or 23 pairs (diploid) in each body cell. In forming the gametes (egg or sperm), one of each chromosome pair will be given, so these cells have only 23 single chromosomes (haploid). In this way, the father contributes half of the genetic information (genotype) for the child; the mother will contribute the other half.

The position of each and every gene is in the exact corresponding location on the same chromosome of every organism within a particular species. Even closely related people look slightly different from each other because of alleles. Alleles are different forms of a gene that controls a trait. Some alleles are dominant and some are recessive. Recessive alleles are only expressed if the homozygous recessive genotype is present. Genotype refers to the actual DNA sequence, or actual alleles present, whereas phenotype is the visible expression of the genotype. An individual with two of the same allele for a given gene is homozygous, and one with two different alleles for the same gene are heterozygous.

Both detached earlobes and dimples are considered dominant traits.

Human Trait
Dominant Allele
Recessive Allele
Dimples
Have dimples
No dimples
Earlobes
Detached
Attached
Freckles
Freckles present
No freckles

Notes:
  • Dimples are natural dents in the face to the right or left of the mouth. If a person has only one dimple, they should be counted as having dimples.
  • A person with attached earlobes will have the lowest point of the earlobe attached to the face. A person with unattached earlobes will not have the lowest point of the earlobe attached to the face. In fact, you may be able to put part of your fingertip between the face and the unattached bottom portion
    of the ear lobe.
  • Tongue-rolling is not a simple inherited trait. The genetics behind tongue rolling are not very secure.

The chances of having a child with dominant trait depend on whether the parents are heterozygous or homozygous. If both the parents are heterozygous for detached earlobes, the analysis is as follows:

Let D = detached earlobes, d = attached earlobes.

Crossing D/d with D/d give the following Punnett square:

D
d
D
D/D
D/d
d
D/d
d/d

There is one chance in four of getting attached earlobes. This is the characteristic 3:1 ratio among offspring which indicates a monohybrid cross between two heterozgyotes.

If a woman is homozygous for detached earlobes, she is therefore D/D. If a man has attached earlobes, the only way for him to be that way is if he has two recessive alleles: d/d. Crossing D/D with d/d gives the following Punnett square:

D
D
d
D/d
D/d
d
D/d
D/d

The chances of having children with detached earlobes are 100%. Genotypically, each child would be a carrier (i.e. heterozygous).

For two parents both with detached earlobes having a child who has attached earlobes, the analysis is as follows:

Crossing detached earlobes with detached earlobes give the following Punnett square:

D
?
D
D/D
D/?
?
D/?
?/?

Child with attached earlobes would be dd. Therefore each parent would be heterozygous D/d.

With similar analysis, if both parents were homozygous recessive, they could not have a child with the dominant allele.

Question Asked By:

Name: karen
Age Group: 21 to 30
Occupation Type: Educationalist
Education Level: Graduate

 
 

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