Topic 3 hard

Question 1

Law of Segregation:

Answer 1

When gametes form, alleles are separated so that each gamete carries only one allele for each gene

Question 2

Law of Independent Assortment:

Answer 2

The segregation of alleles for one gene occurs independently to that of any other gene*

The law of independent assortment does not hold true for genes located on the same chromosome (i.e. linked genes)

Question 3

Principle of Dominance:

Answer 3

Recessive alleles will be masked by dominant alleles†

† Not all genes show a complete dominance hierarchy – some genes show co-dominance or incomplete dominance

Question 4

What are menders laws of inheritance?

Answer 4

1. law of segregation
2. law of independent assortment
3. principle of dominance

Question 5

how are gametes formed

Answer 5

by the process of meiosis

Question 6

are gametes haploid or diploid?

Answer 6

haploid, this means they only possess 1 allele of each gene

Question 7

the combination of alleles can be classified as:

Answer 7

homozygous
heterozygous
hemizygous

Question 8

hemizygous

Answer 8

males only have 1 allele for each gene located on a sex chromosome

Question 9

genotype is typically either

Answer 9

homozygous or heterozygous

Question 10

how are human blood cells categorised?

Answer 10

based on the structure of a surface glycoprotein (antigen)

Question 11

when will an autosomal recessive disease occur?

Answer 11

if both alleles are faulty

Question 12

example of an autosomal recessive genetic disease

Answer 12

cystic fibrosis

Question 13

when will an autosomal dominant genetic disease occur?

Answer 13

when there is one copy of a faulty allele

Question 14

example of an autosomal dominant genetic disease

Answer 14

Huntington’s disease

Question 15

genetic disease is caused by co-dominant alleles it will also only require….

Answer 15

1 copy of a faulty allele to occur

- However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele

Question 16

a genetic disease that displays co-dominance

Answer 16

sickle cell anaemia

Question 17

Cystic Fibrosis

Answer 17

Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
Individuals with cystic fibrosis produce mucus which is unusually thick and sticky
This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
Heterozygous carriers who possess one normal allele will not develop disease symptoms

Question 18

Huntington’s Disease

Answer 18

Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)
More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats)
When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration
This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years)
Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia

Question 19

sex linkage

Answer 19

refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)

Question 20

are sex-linked conditions usually X or Y linked

Answer 20

X linked

Question 21

why are sex-linked conditions usually X linked?

Answer 21

very few genes exist on the shorter Y chromosome

Question 22

Why do sex-linked inheritance patterns differ from autosomal patterns

Answer 22

Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY)

This leads to the expression of sex-linked traits being predominantly associated with a particularly gender

Question 23

why are X linked dominant traits more common in females

Answer 23

females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous
X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)

Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits

X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele

Question 24

The following trends for X-linked conditions:

Answer 24

Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers
Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)

Question 25

examples of sex-linked inheritance

Answer 25

red-green colour blindness

haemophilia

Question 26

assigning alleles for a sex-linked trait

Answer 26

the convention is to write the allele as a superscript to the sex chromosome (X)

Haemophilia: X^H = unaffected (normal blood clotting) ; X^h = affected (haemophilia)
Colour blindness: X^A = unaffected (normal vision) ; X^a = affected (colour blindness)

Question 27

Haemophilia

Answer 27

Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired

The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time
Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.g. haemophilia A = factor VIII)

Question 28

Red-Green Colour Blindness

Answer 28

Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues

This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome
Red-green colour blindness can be diagnosed using the Ishihara colour test

Question 29

A gene mutation

Answer 29

a change to the base sequence of a gene that can affect the structure and function of the protein it encodes

Question 30

how can gene mutations be caused

Answer 30

can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements

Question 31

what is a pedigree?

Answer 31

a chart of the genetic history of a family over several generations

Question 32

pedigree chart features

Answer 32

Males are represented as squares, while females are represented as circles
Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected
A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line
Generations are labeled with roman numerals and individuals are numbered according to age (oldest on the left)

Question 33

how can Dominant and recessive disease conditions may be identified?

Answer 33

if certain patterns occur (otherwise it cannot be confirmed) - (maybe can be determined through using pedigree charts)