Inheritance

Question 1

What does the law of independent assortment say about the segregation of unlinked genes?

Answer 1

• Pairs of alleles are inherited independently of one another if their gene loci are on two separate chromosomes. Genes are unlinked.
• This occurs due to random orientation of homologous pairs during metaphase 1
• Independent segregation of unlinked genes results in a greater number of potential gamete combinations (more phenotypes too)

Question 2

How do you construct a dihybrid cross?

Answer 2

• Potential gamete combinations
• Genotypes and phenotypes of parents
• Use large punnet square with 16 boxes
• Phenotype ratios (9:3:3:1)
• Write down genotype (AaBb) and phenotype of offspring (wrinkled and big)

Question 3

How yo you calculate the genotypic and phenotypic ratio of offspring of dihybrid crosses?

Answer 3

• 9:3:3:1 between two heterozygous parents if they are unlinked genes and show independent assortment
• Simplify the questions asked on this. Sometimes just use a monohybrid cross

Question 4

What are linked genes?

Answer 4

• A set of genes at different loci on the same chromosome
• The closer the loci of two linked genes, less frequently are separated by crossing over
• Linked genes inherited together, do not follow dihybrid cross inheritance

Question 5

What is a linkage group?

Answer 5

• Several genes on the same chromosome that do not show independent assortment
• Phenotypic ratio aligned to a monohybrid cross, genes inherited as single unit
• Linked genes can be separated by crossing over during meiosis 1

Question 6

Why do many linked genes give the same results as unlinked genes in genetic crosses?

Answer 6

• An allele inherited for one gene does not affect which allele will be inherited for the other gene
• The inheritance of the two traits are randomized since crossing over occurs frequently

Question 7

What did Morgan discover using fruit flies (Drosophila)?

Answer 7

• Linked genes are not independently assorted
• Resulted in non-Mendelian ratios

Question 8

What did Morgen discover in regard to sex linkage in fruit flies?

Answer 8

• When red-eyed wild types and white-eyed mutants were cross breed, there was a sex bias in phenotypic distribution
• All female offspring of a red-eyed male were red-eyed, all male offspring of a white-eyed female were also white-eyed
• Sex-limited inheritance, gene of eye colour on sex chromosome (X-linked)

Question 9

To what conclusion did Morgen come when the different traits did not conform Mendelian ratios?

Answer 9

• The alleles for these traits were located on a shared chromosome (gene linkage), no independent assortment
• Linked alleles could be uncoupled by recombination (crossing over) to create alternative phenotypic combinations, new phenotypes would occur at low frequency (very little crossing over)

Question 10

Why does the amount of crossing over depend on the combination of traits?

Answer 10

• Crossover frequency may be a product of the distance between two genes on a chromosome.
• Genes further away have a higher chance of crossing over, and genes closer together have a lower change of crossing over
• Established linkage maps to show the relative positions of genes on a chromosome
  Check book

Question 11

What are recombinations and when does it occur?

Answer 11

• Crossed over chromosomes that result from crossing over of genetic material during prophase 1 of meiosis
• Occurs in linked genes
• They are not found in parents (happens after fertilization)
• Linked genes separated by a chiasma, exchange of alleles between the non-sister chromatids
• New allele combinations

Question 12

Do recombinant or non-recombinant phenotypes have a higher frequency and why?

Answer 12

• There will be less recombinant phenotypes and more non-recombinant phenotypes
• Crossing over is random (happen 10% of the time)

Question 13

On what does the relative frequency of recombinant phenotypes depend on?

Answer 13

• Distance between linked genes
• Frequency is greater when genes are further apart on the chromosome and vice versa
• Since there are more possible locations a chiasma could form
• Parental combinations are always the most common is offspring

Question 14

Be able to identify the possible recombinant phenotypes on a test cross.

Question 15

What are the two types of variations? Explain them.

Answer 15

• Discrete and continuous
• Discrete: a trait with phenotypes that can be grouped into distinct categories, code for a single gene
• Continuous: unbroken range of phenotypes in the population, there can be an intermediate between two phenotypes (polygenic traits)

Question 16

What are the two types of traits? Explain them.

Answer 16

• Monogenic traits: characteristics controlled by a single gene loci, exhibit discrete variation, only expressing one distinct phenotype
• Polygenic traits: characteristics controlled by more than two gene loci, exhibit continuous variation, phenotype falls along a continuous spectrum of potential phenotypes

Question 17

Explain polygenic inheritance in more detail.

Answer 17

• Increasing the number of loci responsible for a particular trait, increases the number of potential phenotypes
• Results in a phenotypic distribution, follows a bell-shaped distribution curve
• E.g. height, skin colour, fur colour

Question 18

What environmental factors can influence polygenic traits?

Answer 18

• Human height for instance can be affected by diet and health (disease), freckles affected by sun exposure
• Skin colour can be controlled by multiple genes but also by factors such as sun exposure

Question 19

What is the chi-squared test? How do you perform it?

Answer 19

• Assess the degree of difference between observed results and the expected results
  Steps explained in book