8 - Human Genetic Variation

Question 1

Small genotypic differences can result in

Answer 1

big phenotypic differences

Question 2

How can small genotypic differences can result in big phenotypic differences

Answer 2

• Due to differences in how those genotypic differences are patterned and/or where they are located in the genome
• If genetic sequences are under selection
• When genes are pleiotropic
• If differences are concentrated in regulatory genes

Question 3

Human variation

Answer 3

• Genetic variation (differences in genotype)
• Environmental variation (due to habitat, climate, culture)

Question 4

Traits influenced by single genes

Answer 4

• If a trait is suspected of being influenced by a
  single gene (e.g. pea colour), then we can more easily separate those effects from
  environmental variation
• They will follow standard patterns of Mendelian inheritance

Question 5

Specific Language Impairment (SLI)

Answer 5

• Children have difficulty learning to speak
• Dominant gene, most are heterozygotes
• e.g. caused by FOXP2 gene

Question 6

FOXP2 gene

Answer 6

• Single dominant allele
• Codes for transcription factor that regulates gene expression in the brains of the foetus

Question 7

Two groups variation can be separated into

Answer 7

• Variation within groups
• Variation between groups

Question 8

Causes of genetic variation within groups

Answer 8

• Mutation can maintain deleterious genes at a low frequency.
• If mutation rates introduce deleterious alleles at the same rate that selection removes them, a selection-mutation balance is said to
  occur.
• e.g. CF still exists because caused by recessive allele. Most people with the gene are heterozygotes and likelihood of that allele being expressed is relatively low

Question 9

Second cause of genetic variation within groups

Answer 9

Balanced polymorphism when heterozygotes have higher fitness than either homozygote then both alleles can be maintained through selection

Question 10

Example of balanced polymorphism

Answer 10

• Sickle-cell anemia
• Homozygote recessive individuals have abnormal red cells
• Heterozygous individuals are protected against malaria

Question 11

Causes of genetic variation between groups

Answer 11

• Selection
• Genetic drift

Question 12

Selection between groups

Answer 12

• Favours different genes in different environments
• Can maintain variation among groups because humans live in many different environments
• E.g. Lactose persistence and Sickle cell allele virtually unknown in some groups

Question 13

Lactose persistence

Answer 13

• Mammals drink milk as infants, but lose ability to digest lactose as adults
• Some people can digest lactose as adults because they still produce lactase in their digestive tracts
• Lactase persistence controlled by a single gene with two alleles (lactase is dominant)

Question 14

Lactose

Answer 14

Sugar in milk

Question 15

Lactase

Answer 15

Enzyme that breaks down lactose