Mendelian genetics

Question 1

What is a monohybrid cross

Answer 1

• breed together two homozygous (pure bred) strains with a single contrasting trait
• F1 generation: one characteristic disappears entirely
• F2 generation: both characteristics seen in 3:1 ratio

Question 2

What is Mendels postulates today for a monohybrid cross

Answer 2

i. in a diploid individual, genes have two alleles, each on a chromosome of the homologous pair
ii. in the pair of alleles for a single characteristic in an individual, one allele is dominant the other is recessive
iii. the two alleles of a single gene separate independently during gamete formation

Question 3

Monohybrid ‘rules’

Answer 3

• if there is a recessive phenotype the genotypes must be homozygous
• for a dominant phenotype a cross-test will show if its a homozygous or heterozygous genotype

Question 4

What is a dihybrid cross

Answer 4

• two contrasting characteristics are crossed (independent traits assumed)
• F1 generation: all show dominant traits
• F2 generation: ratio of 9:3:3:2 seen

if traits were segregated together 9:3:3:1 would not be observed either 3:1 or 2:1:1 depending on parental combination

Question 5

What is Mendels postulates today for a dihybrid cross

Answer 5

i. traits sort independently during gamete formation
ii. all possible combination of gametes will form with equal frequency

Question 6

How do Mendels results relate to chromosome behaviour during meiosis

Answer 6

• when there are two characteristics controlled by two genes on different chromosomes the gametes formed are the same Mendel predicted int eh dihybrid cross
• independent assortment of the chromosomes at meiosis explains Mendels ratio for the dihybrid cross

Question 7

What the method for genetic crosses

Answer 7

• forked-line method

using ratios to work out the probability of genotypes and phenotypes forming