chapter 8: patterns of inheritance

Question 1

explain what a pedigree chart is

Answer 1

A pedigree uses symbols to show the inheritance of a particular trait.

can indicate if

• the trait concerned is dominant or recessive
• the controlling gene is located on an autosome or on a sex chromosome.

Question 2

describe features of pedigree for an autosomal dominant trait

Answer 2

• Both males and females are affected in broadly equal numbers across a large pedigree. a
• All affected individuals have at least one affected parent. d
• Once the condition disappears from a branch of the family tree, it never reappears. d
• Fathers can pass the condition on to both sons and daughters, as can mothers. a

Question 3

describe features of a pedigree for an autosomal recessive trait

Answer 3

• Both males and females are affected in broadly equal numbers across a large pedigree. a
• Two unaffected parents can have an affected child. (the parents are carriers) r
• parents who are both affected will have children who all have the condition r
• The trait can ‘skip’ a generation. In other words, it may disappear from a branch of the pedigree and reappear in later generations. r

Question 4

describe features of a pedigree for an X-linked dominant trait

Answer 4

• A male with the trait passes it on to all his daughters and none of his sons. x
  
  • daughter gets one x from father and one x from mother and bc the trait is dominant, having one dominant allele is enough for daughter to express the trait
  • none of the father’s son inherits the trait because males get their x chromosome from their mothers whilst they their y chromosomes come from their fathers
• Every affected individual has at least one parent with the trait. d
• Once the trait disappears from a branch of the family tree, it never reappears. d
• In a large pedigree there are more affected females than males. x

Question 5

describe features of a pedigree for an X-linked recessive trait

Answer 5

• Affected females always produce affected sons.
  
  • affected female would have two of the recessive alleles, and as males get their x chromosome from their mothers, they will inherit the recessive allele, and they only need one recessive allele to inherit the trait because males only have one x chromosome not two
• The daughters of affected males are always carriers.
  
  • get one x chromosome from mum and one from dad, would get the recessive allele from dad and therefore is a carrier
• All the children with two affected parents will have the condition.
• Across a large pedigree, many more males than females will be affected.

Question 6

use pedigrees to determine the genotype

Answer 6

• can use pedigrees to determine the genotypeto determine if it is autosomal or sex-linked
  • affects both males and females in relatively equal amounts → autosomal
  • affects a specific sex more so than the other one → sex-linked
  to determine if dominant or recessive:
  • doesn’t skip generation: all affected children have at least 1 parent with the trait → dominant
  • can skip generations: unaffected parents can produce affected children → recessive

Question 7

what is a test cross

Answer 7

a cross used to determine the genotype of an individual with an unknown genotype by crossing it to an individual with a homozygous recessive genotype

Question 8

what is a monohybrid cross

Answer 8

a cross in which alleles of only one gene are involved

Question 9

what is a dihybrid cross

Answer 9

a cross in which alleles of two different genes are involved

Question 10

what are parental (non-crossover) gametes - form

Answer 10

contains the combination of alleles that were initially located on the same chromosome (as present in the parental cells)
identical to the combination of allele in the parents

Question 11

what are recombinant (crossover) gametes

Answer 11

• forms when linked genes are not inherited together
• sex cells that contain new combinations of alleles that occur due to crossing over
• they do form but not very often

Question 12

use a punnet square to predict the outcomes of a monohybrid cross in autosomes

Answer 12

1. parent genotypes
2. possible parent gametes
3. punnet square
4. write outcomes

Question 13

what are linked genes

Answer 13

• linked genes are on the same chromosome
• crossing over would prevent genes on the same chromosome from ending up in the same gametes

Question 14

calculate map units between linked genes

Answer 14

distance between linked genes in map units= 100 x number of recombinant offsprings/total number of offsprings

so if there is 12% recombinant offsprings, the genes are 12 map units apart

Question 15

the likelihood of recombinant offspring

Answer 15

• the likelihood of crossing over forming recombinants depends on how close the genes are
• if genes are on the opposite ends of chromosomes, crossing over is more likely to separate them
• the closer the genes are the less likely they will be separated by crossing over

Question 16

significance of linkage

Answer 16

• when scientists discover a new mutation, they can look for linkage to other genes
• this can determine the location of the mutation on a chromosomes and help identify mutated gene

Question 17

expected outcome of a dihybrid unlinked cross

Answer 17

The expected outcome of a dihybrid cross of two heterozygotes is four phenotypes in the ratio 9 : 3 : 3 : 1.

Question 18

how to determine whether two genes are linked or not

Answer 18

if the ratio of offsprings differ to that of the expected ratio
eg. if two heterozygotes are crossed, the ratio should be 9:3:3:1, however, if that ratio is different, then the 2 genotypes with a larger number of offsprings would be the parental form and the other 2 would be the recombinants

Question 19

define purebred and hybrid

Answer 19

• purebred: means that the plant is homozygous for the allele
• hybrid: a cross of two parents