10.1

Question 1

Crossing over

Answer 1

• during prophase 1 of meiosis 1 homologous chromosomes pair up to form bivalents in a process called synapsis
• non-sister chromatids break and redoing with each other at points called chiasmata, which allows for the exchange of DNA between chromatids
• chiasmata hold chromatids together until the end of prophase 1 where they slide to the end of a bivalent allowing homologous chromosomes to separate

results in new combination of alleles on chromosomes (called recombinants

Question 2

loci affecting inheritance probability

Answer 2

the closer together genes are located on a chromosome the less likely chiasmata will form between them, so the more likely they are to be inherited together

Question 3

independent assortment of genes

Answer 3

• in metaphase 1 of meiosis 1, bivalents line up on the equator of a cell
• the orientation of bivalents on the metaphase plate is random and independent of other bivalents
• this means that there is a 50:50% chance of each allele moving to a particular pole of the cell when homologous chromosomes separate during anaphase 1

Question 4

What theory did Thomas hunt Morgan develop

Answer 4

gene linkage:
- genes located on the same chromosome as each other
- these aren’t independently assorted into gametes during meiosis
- they are likely to be inherited together
- generates a high proportion of parental phenotypes
- recombinants are rare because they’re a result of crossing-over

Question 5

draw diagrams to show chiasmata formed by crossing over

Answer 5

Question 6

What did Thomas Hunt Morgan find

Answer 6

• Morgan found non-mendelian ratios in Drosophila melanogaster
• he crossed normal red-eyed flies with a mutant white eyed fly
• a 3:1 ration of the phenotypes in f2 is what is predicted
• however, Hunt found that all of the white-eyes flies in f2 were male whereas red-eyed flies were both male and female
• therefore the gene for eye colour is sex linked and white eyed males X^b Y need only one copy of the recessive allele to have white eyes

Question 7

Variation

Answer 7

can be discrete:
- when a characteristic falls into one of a number of distinct categories
- eg. purple vs red flowers, green vs yellow peas, smooth vs wrinkled peas
NB: controlled by single genes w/ different alleles

can be continuous:
- when a characteristic can take any value in a range
- eg. eye colour, natural hair colours, skin colour, weight, wing span
NB: all controlled by several genes working together

Question 8

Polygenic characteristics

Answer 8

• are controlled by 2 or more different genes
• the phenotype produced is determined by the collective effect of all the genes