WEEK 10

Question 1

Gene

Answer 1

A section of DS DNA that encodes a particular trait, influences/determines a characteristic

Question 2

Allele

Answer 2

An alternate form of a gene

Question 3

Locus (loci when plural)

Answer 3

The physical location of a gene on a chromosome

Question 4

Genotype

Answer 4

The genetic composition of an individual, the set of alleles an individual possesses (two alleles)

Question 5

Phenotype (trait)

Answer 5

The physical expression, appearance or manifestation of a gene or allele

Question 6

Dominant allele

Answer 6

Expressed whether alone or as a pair (symbolised by a capital letter)

Question 7

Recessive allele

Answer 7

Expressed only in the absence of a dominant allele (Symbolised in lowercase)

Question 8

Homozygous

Answer 8

An individual possessing a pair of the same alleles at a locus (i.e AA or aa)

Question 9

Heterozygous

Answer 9

An individual possessing two different alleles (i.e Aa)

Question 10

Somatic cells

Answer 10

Diploid: 46 chromosomes, 22 pairs autosomes, 1 pair sex chr (23rd pair)

Question 11

Gametes

Answer 11

Haploid: only one chr set (23)

Question 12

Zygote

Answer 12

Diploid: 23 paternal + 23 maternal chr

Question 13

Define Monogenic

Answer 13

Affects a single gene

Question 14

Monogenic Disorder Inheritance

Answer 14

Can be; Autosomal or Sex-linked
- Autosomal (Recessive or Dominant)
- Sex-linked (Y-linked or X-linked)
- X-linked (Recessive or Dominant)

Question 15

Law of Segregation (Mendel)

Answer 15

For Monogenic traits:
- each individual has two alleles for the trait (autosomal)
- genes segregate during gametogenesis
- fertilisation gives each new individual two genes, one from each parent
- ploidy level of gametes: 2n (diploid)
- ploidy level after fertilisation: n (haploid)

Question 16

Mendel’s first law - segregation of alleles

Answer 16

• cells contain 2 copies (alleles) of each gene
• alleles separate during gamete formation (meiosis)
• gametes carry only one copy of one gene (random)

Question 17

Dominance

Answer 17

In a heterozygote, one allele may conceal the presence of another

Question 18

Why is human genetic analysis complex

Answer 18

• incomplete family records
• small number of progeny
• uncontrolled environment

Question 19

Types of inheritance of Monogenic disorders

Answer 19

• autosomal recessive (Cystic Fibrosis)
• autosomal dominance (Huntington’s disease)
• sex-linked recessive (Haemophilia)
• sex-linked dominant (Rickets)

Question 20

Inheritance of a Recessive Trait

Answer 20

• recessive traits may occur in individuals whose parents are not affected
• must be homozygous recessive to express trait
• if heterozygous for the trait = carrier

Question 21

Inheritance of a dominant trait

Answer 21

• individuals who carry dominant allele manifest trait
• every affected individual is expected to have at least one affected parent
• if a dominant trait is associated with reduced viability or fertility, most people who show the trait are heterozygous & ~half their children inherit the condition

Question 22

Autosomal Recessive traits

Answer 22

• usually appear equally in male and females
• tend to skip generations
• more likely to appear among progeny of related parents
• affected offspring born to unaffected parents
• when heterozygous parents ~ 1/4 offspring affected

Question 23

Autosomal Dominant traits

Answer 23

• usually appear equally in both sexes
• both sexes can transmit trait to offspring
• does not skip generations
• affected offspring has affected parent (unless new variant)
• unaffected parents do not transmit trait
• if 1 parent heterozygous, 1 unaffected ~ 1/2 offspring affected

Question 24

Sex-linked genes

Answer 24

X or Y chr, these are passed from the parent to offspring
- many genes on X, not Y (hence male are more likely as male only have on X chr)

Question 25

X-linked Recessive traits

Answer 25

- usually more males than females affected (males only 1 X chr, no 2nd X chr to counteract effects of recessive allele) - affected sons usually born to unaffected mothers so skips generations - ~1/2 of heterozygous carrier mother’s sons affected - never passed from father to son - all daughters of affected fathers are carriers

Question 26

X-linked dominant traits

Answer 26

- does not skip generations - both males and females affected, often more females than males - affected males must have affected mother - affected daughters must have affected mother or father - affected fathers will pass trait to all daughters - heterozygous affected mothers will pass trait to ~1/2 sons & ~ 1/2 daughters

Question 27

Law of independent assortment

Answer 27

- each pair of alleles segregate independently of the other pairs - all possible combinations of alleles can occur in the gametes