Lecture 8

Question 1

What is a sex-influenced trait?

Answer 1

the phenotypic expression of the trait is influenced by the sex of the individual

• trait caused by autosomal chromosomes but influenced by the sex chromosome
• see the phenotype in both sexes but one sex has higher frequency than the other

Question 2

What is pattern baldness?

Answer 2

(can be early or late onset)

• sex influenced trait (seen in both sexes)
• much less common in females (appears dominant in males and recessive in females)
• B1B2 male would be bald but B1B2 female wouldn’t
• the baldness mutation leads to over expression of 5 alpha reductase which converts testosterone to DHT= males make more testosterone so the impact is bigger whereas in females it’s not as pronounced
• in women oestrogen influences the B1B2 gene so after menopause more pronounced baldness
• if mother homozygous dominant can pass on baldness onto son

Question 3

What is Polycistic ovary disease?

Answer 3

PCOD- sex influenced trait

• the women who have the baldness mutation produce more testosterone and this is probably involved in the common occurrence of PCOD in these women (increased androgen)
• multiple cysts in the ovary
• up to 12% of childbearing age women have it in Australia
• PCOD and baldness are due to the alleles on the same gene
• differing threshold for phenotypic expression in females and males= explanation of the variable phenotype
• appears to be autosomal dominant with 90% penetrance (10% don’t show any effects)

Question 4

What is breast cancer?

Answer 4

• sex influenced trait
• mostly in females
• two genes identified in the 5% of cancers that are inherited
• genes BRCA 1 mutations limited to females, BRCA 2 expressed in both sexes

Question 5

What is a sex limited trait?

Answer 5

the phenotype is only expressed in one sex

Question 6

What are the two examples of sex limited traits?

Answer 6

e. g. : rooster feathering in fowl
e. g.: precocious puberty in males
- sex limited, only appears in males
- gene mutation which leads to receptor protein on all the time=overactive receptor leads to excess testosterone= early puberty
- puberty in males at the age of 5
- taller than the rest in kindergarten but on average shorter as adults= stop growing earlier than others
- doesn’t interfere with fertility so can be passed on
- father can pass it onto daughter but won’t be expressed= dominant disorder with reduced penetrance

Question 7

How many chromosomes do we have to have for independent assortment to be relevant?

Answer 7

-more than two pairs

Question 8

What is independent assortment?

Answer 8

• basically when have AaBb can end up with AB Ab aB or ab, the pair Aa is independent of Bb and in meiosis they can assort independently
• if large number of gametes are produced assume that all the options happen with the same frequency so 1:1 ratio
• independent in the sense of where they go in meiosis, one doesn’t affect the other

Question 9

How is independent assortment written?

Answer 9

-; semi colon Aa;Bb means that genes are on different chromosomes and are assorted independently

Question 10

How many possible gamete combinations are there?

Answer 10

= 2 to the power of n
=e.g. if 3 pairs (Aa;Bb;Cc) then 2 to the power of three= 8
-in humans 23 pairs so 2 to the 23=8 388 608 possible combinations, that’s why we look so different from our relatives

Question 11

What is a dihybrid cross?

Answer 11

-more than one pair involved, purebreeding parents and then F1 crossed among themselves

F2 Aa;Bb x Aa;Bb

• 4 pair so 16 possible combinations, put all the gametes of one parent on one side and write all the alleles on the same locus so Aa aa, Bb etc but on the sides you’ll have AB ab aB etc…
• ratio of offspring 9:3:3:1 assuming complete dominance of A and B
• this is true for genes that control separate categories

Question 12

What does purebreeding mean?

Answer 12

means homozygous, if self-fertilised then identical offspring to parent
aa AA AA;BB AA;bb aa;BB AA;bb;CC etc

Question 13

What proportion of Pp;Qq;RR;Ss x pp;Qq;Rr;SS offspring will be pp;qq;Rr;Ss?

Answer 13

-look at each locus independently, so each of the letters take separately
so Pp x pp = ½ Pp ½ pp
Qq x Qq = ¼ QQ, ½ Qq, ¼ qq etc and then multiply the probabilities
So ½ x ¼ etc = 1/64

Question 14

What is a test cross?

Answer 14

: -used to cross homozygous recessive genotype
-may be used to establish the genotype of an individual with the dominant phenotype
- to see if the individual is heterozygous
if crossing AA x aa or Aa x aa different outcomes in offspring phenotypic ratios!
-appearance of the recessive phenotype in offspring indicates that the genotype was heterozygous
- doesn’t matter how many gene loci still can do it Aa;Bb;Cc x aa;bb;cc

Question 15

What is a backcross?

Answer 15

a cross of an F1 to one of its parents (or an individual genetically identical)
- useful because you know the genotype of the parent and can use as a constant

Question 16

What is a reciprocal cross?

Answer 16

-to detect of it’s an X linked disease
1st cross -female phenotype A x male phenotype B
2nd cross – female phenotype B x male phenotype A