Topic 14 - Modern Understanding of Inheritance

Question 1

Chromosome Theory of Inheritance

Answer 1

• Chromosomes are the genetic material responsible for Mendelian inheritance
• During meiosis, homologous chromosome pairs migrate independently of other chromosome pairs
• Chromosome sorting from each homologous pair is random
• Each parent synthesizes gametes that contain only half their chromosomal complement
• Male and female gametes have the same number of chromosomes, suggesting equal contributions from each parent
• Gamete chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents

Question 2

Explain the difference in gene ratio for a heterozygote (ex. AaBb) if the genes were linked vs. unlinked.

Answer 2

In this example, the heterozygote inherited dominant maternal alleles for two genes on the same chromosome (A and B) and two recessive paternal alleles for those same genes (a and b).
Parental types: AB and ab
Nonparental (recombinant) types: Ab and aB
- If the genes are linked, one would expect this individual to produce AB and ab gametes with a 1:1 ratio
- If the genes are unlinked, the individual should produce AB, Ab, aB, and ab gametes with equal frequencies (according to independent assortment)

Question 3

What determines the likelihood of crossing over (recombination frequency)?

Answer 3

The distance between the genes on the chromosome

Question 4

How is recombination frequency (rf) calculated?

Answer 4

Recombinants/total offspring

Question 5

The lower the rf, the ______ the genes are

Answer 5

Closer

Question 6

What does an rf of over 50% indicate?

Answer 6

Genes are farther apart; results look like independent assortment

Question 7

How is sex determined in humans? Give examples of how sex is determined in other organisms.

Answer 7

In some species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy.
In humans, the father donates either an X or Y chromosome which determines the sex of the offspring.

Question 8

X-inactivation

Answer 8

Condensing X chromosomes into Barr bodies during embryonic development in females to compensate for double genetic dose

Question 9

Nondisjunction

Answer 9

Failure of synapsed homologues to completely separate and migrate to separate poles during meiosis

Question 10

What causes Down Syndrome? Why does trisomy of large chromosomes lead to nonviable zygotes?

Answer 10

Trisomy 21 causes Down Syndrome.
- Trisomy: gaining an extraneous chromosome
“Gene dosage” determines viability; excess in gene dose usually leads to nonviability with the exception of conditions like Down Syndrome where smaller chromosomes are involved (smaller chromosomes = fewer genes = fewer proteins)

Question 11

What disorders can nondisjunction of sex chromosomes result in?

Answer 11

XXX - triple-X females
XXY males - Klinefelter syndrome
XO females - Turner syndrome
OY - nonviable zygotes
XYY males - Jacob syndrome