Topic 7 - Genetics

Question 1

what is a genotype

Answer 1

the genetic constitution of an organism (its alleles)

Question 2

what is a phenotype

Answer 2

the expression of the genetic constitution and its interactions with the environment (characteristics)

Question 3

what is codominant

Answer 3

alleles that are both expressed in the phenotype (neither one is recessive)

Question 4

what is a locus

Answer 4

the position of a gene on a chromosome

Question 5

what is a homozygote

Answer 5

• 2 copies of the same allele
• homozygote dominant - BB
• homozygote - bb

Question 6

what is a heterozygote

Answer 6

• 2 different alleles
• e.g. Bb

Question 7

what is a carrier

Answer 7

a person carrying an allele which is not expressed in the phenotype but can be passed on to an offspring

Question 8

what is the chromosome for females

Answer 8

XX

Question 9

What is the chromosomes for males

Answer 9

XY

Question 10

what is characteristics of the Y and X chromosomes

Answer 10

• it is smaller than the X chromosome and carries fewer genes
• so most genes on the sex chromosomes are located on the X chromosome (X-linked genes)

Question 11

what is a sex-linked characteristic

Answer 11

• the allele that codes for a characterstic is located on a sex chromosome

Question 12

why do males express recessive sex-linked characteristics

Answer 12

• they only have one X chromosome
• so they express the characterstic of this allele even if its recessive
• this makes males more likely to show recessive phenotypes for genes thar are sex linked

Question 13

what are x-linked disorders

Answer 13

• faulty alleles on a sex chromosome that cause genetic disorders
• colour blindness
• haemophilia
• carried on the X chromosome

Question 14

what is a autosome

Answer 14

chromosome that isnt a sex chromosome

Question 15

why are genes on the same autosome linked

Answer 15

• they stay together during independent segregation of chromosomes in meiosis 1
• and there alleles will be passed on to the offspring together
• the only reason this happens is if crossing over splits them up first
• the closer together the genes the more likely they are to be linked because its less likely to be split in crossing over

Question 16

if two genes are autosomally linked what will happen

Answer 16

• you wont get the phenotypic ratio you expect in the offspring of a cross
• because the two autosomically linked alleles are inherited togetrher

Question 17

what is a monohybrid

Answer 17

• parent with homozygous dominant and homozygous recessive
• RR x rr

Question 18

what is a dihybrid

Answer 18

• parents with homozygous dominant and homozygous recessive
• RRYY x rryy

Question 19

what is epistasis

Answer 19

• many genes interact to form the same phenotype
• this can be because the allele of one gene blocks the expression of the alleles on other genes
• for example in humans a widows peak is controlled by one gene. it doesnt matter if you have the alleles that code for a widows peak if you have the alleles that code for baldness because you have no hair - the baldness gene masks the expression of the widows peak gene