Topic 3.4 Inheritance Patterns

Question 1

Monohybrid Crosses

Answer 1

A monohybrid cross determines the allele combinations for potential offspring for one gene only
→ Crosses can be represented for one gene only

Question 2

Monohybrid crosses are calculated via the following steps:

Answer 2

Designate letters to represent alleles (e.g. A, a).
Identify genotype /. phenotype of parents (P generation).
Determine genotype of gametes (haploid).
Work out gamete combinations with a Punnett grid.
Identify ratios of offspring (F1 Generation).

Question 3

Sex Linkage

Answer 3

Sex linkage refers to when a gene is on a sex chromosome I.e. X or Y (all other chromosomes are autosomal)

Sex Chromosomes (X/Y) 
 → Y chromosome is short and has few genes (\<100)
 → X chromosome is large with many genes (~2000)

Question 4

Sex-Linked Traits

Answer 4

Sex-linked traits have altered inheritance patterns:
→ Males have higher rate of X-linked recessive conditions as the cannot mask the recessive allele (are homozygous)
→ Females can be carriers for X-linked recessive conditions (heterozygotes can carry the allele but not express it)

Question 5

X-Linked conditions

Answer 5

→ Recessive: Affected Mothers must have affected sons

→ Dominant: Affected Fathers must have affected daughters

Question 6

Examples of X-Linked recessive traits include:

Answer 6

→Haemophilia (cannot clot blood properly)

→ Red-green color blindness

Question 7

Modes of Inheritance

Answer 7

A pedigree chart of genetic history over several generations

Question 8

In a typical pedigree chart:

Answer 8

• Males are represented as squares, while females as circles
• Shaded symbols denote individual has specified condition
• A horizontal line between man and women represents mating
• Offspring numbered from left to right according to age

Question 9

Autosomal Dominance

Answer 9

• If both parents are affected by a trait and any offspring is not, the trait must be dominant (parents must be heterozygous).

Question 10

Autosomal Recessive

Answer 10

• If neither parents is affected by a trait but any offspring is, the trait must be recessive (parents must be heterozygous)

Question 11

Sex-Linked Traits

Answer 11

No way to conclusively prove sex-linkage with a pedigree chart, but certain patterns may suggest the possibility.