Lecture 18 - Discussion Questions

What are autosomes? What are sex chromosomes? How does inheritance of genes located on sex chromosomes differ from inheritance of genes on autosomes?

Autosomes are all the chromosomes that are involved in any other function except for sex determination. Two of the chromosomes (the X and the Y chromosome) determine your sex as male or female when you are born. X-linked genes have different inheritance patterns than genes on non-sex chromosomes (autosomes). That's because these genes are present in different copy numbers in males and females. Y chromosomes only receive one allele. Since a female has two X chromosomes, she will have two copies of each X-linked gene.

Which parts of a homologous pair engage in crossing over?

Chiasmata are points where two homologous non-sister chromatids exchange genetic material during crossing over in meiosis.

If two genes are linked and are very close together on the same chromosome, they exhibit dependent assortment. How does dependent assortment affect the gamete genotypes, compared to independent assortment?

Dependent assortment is where all the gametes look like the parental chromosomes and there is no variation, unlike independent assortment.

What are some advantages of using fruit flies as a model organism in genetics?

Fruit flies have a short life span, are raised easily in a lab, and have genetic variation.

Why does a ratio of something other than 1:1:1:1 in a test cross of a double heterozygote (when tracing the inheritance of two genes) indicate linkage?

If the genes are unlinked, then they will assort independently in meiosis and test-cross progeny will be produced in a 1:1:1:1 ratio. If the two genes are linked, then more than 50% of the progeny will display parental phenotypes and less than 50% of the progeny will display recombinant (nonparental) phenotypes. Dihybrid - 9:3:3:1

Explain why linkage is a violation of independent assortment, and how that affects the frequency of the resulting gamete genotypes.

Linkage is a violation of independent assortment since the genes are located on the same chromosome. There will then be only one option for the gametes alleles since they are connected.

How does linkage notation work? Why do we use a different notation with linked genes?

Linkage notation (AB/ab) to represent the alleles on the same gene.

Why do males (individuals who have XY sex chromosomes) exhibit X linked recessive diseases at a much higher frequency than females (individuals with two X chromosomes)?

Males exhibit X linked diseases at a much higher frequency since they only receive one allele on the X chromosome.

How do you know which gametes or offspring should be designated as parental and which gametes should be designated as recombinant?

Parental gametes will have the same alleles as the parents (ex. AB or ab), while recombinant types will have one allele from each chromosome (ex, Ab or aB).

Why would a geneticist do a reciprocal cross? What does it tell us?

Reciprocal cross would predict the different variation possible in a cross. If autosomal cross, results are the same. If X-linked cross, there are different outcomes.

How do we know the human X and Y chromosomes are a homologous pair?

Sequence similarity between X and Y chromosomes.

Describe how TH Morgan figured out the white gene (that codes for eye color in fruit flies) was located on the X chromosome. To do this, trace the logic of the crosses he did.

TH Morgan first crossed two true-breeding red-eyed and white-eyed flies which created an F2 generation of red-eyed females and males. Two F2 flies then crossed resulting in all females having red eyes and some males having red while the others white. The white eyes appeared only in males since the white allele is recessive and expressed on the X chromosomes.

How do we use information on a genetic map? How do you use a genetic map to determine whether a specific gamete or offspring phenotype will be common or rare?

The distance between adjacent genes is given in map units, which equal to the frequency of recombination when expressed as a percentage. The farther the genes, the more likely recombination will occur.

Describe a breeding experiment you could do to determine if two genes are linked.

To tell is two genes are located on the same chromosomes, cross a homozygous dominant with a homozygous recessive. The cross a F! with a homozygous recessive (test cross). If expected offspring proportions do not match observed, then you can infer the two genes are located on the same chromosome.

What is the notation for 2 genes both located on the X chromosome?

X^AB