Chromosomal Basis of Inheritance

Question 1

Explain study by Thomas Hunt Morgan that involves Drosophila melanogaster (fruit fly)

Answer 1

He found a male with a white eyes, mated it with a female with red eyes, F1 generation all had red eyes, F2 generation showed 3 red: 1 white (males had white)
Fly’s eye color linked to biological sex
Male flies get X chromosome from their mother who is heterozygous for red eye leading to fifty-fifty split of phenotypes

Question 2

Wild-type

Answer 2

Most common character phenotype

Question 3

Mutant

Answer 3

Less common character phenotype

Question 4

Recombinant phenotype

Answer 4

Occurs when crossing over occurs
If offspring possess a non-parental phenotype

Question 5

Linked genes

Answer 5

Do not assort independently of each other
Travel together during meiosis and fertilization

Question 6

Unlinked genes

Answer 6

Assort independently of each other
Not near each other on the same chromosome or found on separate chromosomes (crossing over occurs at prophase l between homologous chromosomes, more likely when genes are far apart)

Question 7

Incomplete linkage

Answer 7

Give rise to recombinants
Genes present on a chromosome more likely to cross over

Question 8

Why are males more likely to inherit sex-linked disorders compared to females

Answer 8

Males only have one X chromosome, so if they receive the recessive trait from the mother they would have the sex-linked disorder
Females can be heterozygous so they are carriers

Question 9

Examples of human sex-linked disorders

Answer 9

Duchenne muscular dystrophy - due to absence of X-linked gene for muscle protein, dystrophin (individuals rarely live past early 20’s)
Hemophilia - absence of one or more clotting factors (bleeding in muscles and joints)
Hypertrichosis - can be present at birth or acquired later in life (excessive or animal-like hair on face and body)

Question 10

Non-disjunction

Answer 10

Pairs of homologous chromosomes do not separate normally during meiosis 1 or 2
Gametes contain 2 copies or no copies

Question 11

Aneuploidy

Answer 11

Occurrence of extra or missing chromosome due to non-disjunction, which leads to an unbalanced chromosome complement

Question 12

Monosomy

Answer 12

XO or X- (Turner’s syndrome)

Question 13

Trisomy

Answer 13

Down syndrome - 3 copies of chromosome 21
XXX - trisomy X
XYY - Jacob’s syndrome
XXY - Klinefelter’s

Question 14

Examples of human aneuploid conditions

Answer 14

Down syndrome - due to 3 copies of chromosome 21
Klinefelter’s syndrome - XXY (male sex organs, female characteristics, sterile, normal intelligence)
Monosomy X or Turner’s syndrome - XO or X- (short stature, sterile, normal intelligence)
Trisomy X - (XXX) healthy females with symptoms of developmental delay
Jacob’s syndrome - (XYY) taller than average, below normal intelligence, still able to have children

Question 15

Cri du chat

Answer 15

Human syndrome based on structural alteration in one of the chromosomes (specific deletion in chromosome 5)
Not an aneuploid condition
Distinct facial features, severe cognitive speech, high pitch cry like distressed cat