chromosome behaviour during cell divisions

Question 1

what occurs in each of the following sections of prophase I: leptotene, zygotene, pachytene, diplotene

Answer 1

leptotene: chromosomes start to condense
zygotene: homologous chromosomes become closely associated to form pairs of chromosomes consisting of 4 tetrads
pachytene: crossing over between pairs of homologous chromosomes to form chiasmata
diplotene: homologous chromosomes continue to separate but remain attached by chiasmata

Question 2

what is meant by the term ‘cross’?

Answer 2

the fusion of male gametes from one individual and female gametes from another

Question 3

define ‘monohybrid cross’

Answer 3

crosses between true breeding strains with alternative forms of a single trait

Question 4

what information does a reciprocal cross give you?

Answer 4

whether inheritance of a trait depends on sex

Question 5

what is Mendel’s principle of segregation?

Answer 5

alleles separate from eachother during the formation of gametes in meiosis

Question 6

what is meant by product rule?

Answer 6

the probability of two interdependent events occurring simultaneously, the product of each of their individual probabilites

Question 7

what is meant by sum rule?

Answer 7

the probability of occurance of any mutually exclusive events is the sum of the probability of individual events

Question 8

what is the Chi-squared test used for?

Answer 8

to determine whether a difference is due to chance, and if the null hypothesis should be rejected

Question 9

what is meant by complementation?

Answer 9

when 2 strains of an organism with different homozygous recessive alleles produce offspring with wild-type phenotype when crossed

Question 10

why does linkage happen?

Answer 10

alleles on the same chromosome will not assort independently and will co-segregate at meiosis

Question 11

what indicates linkage?

Answer 11

greater observed number of parental phenotypes than recombinants or by RF <50%

Question 12

how is RF calculated?

Answer 12

total recombinants/total progeny x100

Question 13

what causes RF to reach 50%?

Answer 13

one or more chiasmata between 2 genes in every meiosis

Question 14

what does a dominance test test?

Answer 14

studies the phenotype of the heterozygote F1; the functional relationship of alleles on the same gene

Question 15

what does a complementation test study?

Answer 15

the phenotype of F1, tests the functional relationship of two recessive mutations

Question 16

what are the characteristics of an autosomal recessive disease?

Answer 16

• heterozygotes unaffected
• 2 heterozygotes have 25% chance of having affected offspring
• 2 homozygous individuals will have affected offspring only
  e. g: cystic fibrosis

Question 17

what are the properties of an autosomal dominant disease?

Answer 17

• every affected individual has 50% chance of having affected offspring
• disease cannot skip generations
• e.g. Huntington’s disease

Question 18

what are the properties of an X-linked recessive disease?

Answer 18

• hemizygous males and homozygous females affected
• female carriers have 50% chance of having affected sons
• affected males cannot give traits to sons
• e.g. Haemophilia A

Question 19

what are the properties of an X-linked dominant disease?

Answer 19

• hemizygous males and heterozygous females affected
• affected male cannot give trait to sons but gives it to his daughters
• focal dermal hypoplasia