Understanding Chromosomes, Inheritance, and Genetic Traits
Similarities and Differences in Homologous Chromosomes
Prior to crossing over, in what ways are homologous chromosomes similar? In what ways do they differ?
- Homologous chromosomes are chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location.
- They have the same genes but not necessarily the same alleles. For example, they could both carry the gene for hair color, but one might have the allele for brown hair while the other has the allele for blonde hair. Therefore, homologous chromosomes are similar but not identical. Each carries the same genes in the same order, but the alleles for each trait may not be the same.
Heterozygous vs. Homozygous
The term heterozygous can be used on its own, but the term homozygous should be followed by either recessive or dominant. Explain why.
A heterozygous individual carries both dominant and recessive alleles but only shows the dominant phenotype. A homozygous individual carries a “pure” genotype of either the recessive or the dominant trait. Therefore, it’s necessary to specify whether the individual is homozygous dominant or homozygous recessive.
X-linked Severe Combined Immunodeficiency (SCID)
A man with X-linked Severe Combined Immunodeficiency (SCID) passes this trait onto all of his female offspring.
Because a male can pass down either an X or a Y chromosome, all of his female children will inherit his X-linked traits.
Are Human Sex Chromosomes a Homologous Pair?
The sex chromosomes in a human are a homologous pair. Do you agree or disagree with this statement? Explain.
While technically you receive one sex chromosome from each parent, X and Y chromosomes differ significantly.
X and Y chromosomes have:
- Different sizes
- Different genes on each
Therefore, while they pair during meiosis, they are not truly homologous in the same way as autosomes.
Non-homologous Chromosome Synapsis During Meiosis
What would happen if a chromosome were to synapse with a non-homologous chromosome during meiosis, rather than with its homologue?
A disorder or mutation could potentially arise in the phenotype.
Why Males Cannot Be Carriers of X-linked Traits
Explain why males cannot be carriers of X-linked traits.
Males cannot be carriers of X-linked traits because they only have one X chromosome, whereas females have two. If a male inherits an X-linked recessive trait, he will express it because he doesn’t have another X chromosome to potentially mask the trait.
Understanding the Probability of Albinism
In humans, albinism is recessive to normal pigmentation. Two of your friends have normal pigmentation. They have one child who is albino and are expecting a second child. Your friends tell you, “We knew that our chance of having one albino child is 1:4, so we are sure that our next baby will not be albino.”
How would you respond to your friends? Please write a paragraph.
“I understand your reasoning, but unfortunately, each child’s inheritance is an independent event. The probability of your second child having albinism is still 1:4, or 25%. While the odds favor your child having normal pigmentation, there’s still a chance they could inherit the recessive alleles for albinism from both of you.”
Klinefelter’s Syndrome: A Genetic Disorder
Earlier in this unit, you researched and summarized a particular genetic disorder/syndrome that resulted from nondisjunction. Name this syndrome, explain how it occurs, and describe the symptoms/characteristics of an individual that has the syndrome.
Klinefelter’s syndrome is a genetic disorder that occurs in males due to nondisjunction, resulting in an extra X chromosome (XXY). Individuals with this disorder tend to be taller than average. They often have lower testosterone levels, leading to reduced muscle mass and decreased facial and pubic hair growth.
Inheritance of Red-Green Colorblindness
Parents who do not have red-green colorblindness have a son with red-green colorblindness. Which parent has passed on this condition to their son?
The mother has passed on this condition to her son because red-green colorblindness is an X-linked recessive trait. This means the mother was a carrier of the trait, and only the mother can pass it on to her male offspring.