Genetic Inheritance Of Disease

Question 1

Define genetic diversity

Answer 1

genetic diversity refers to the differences in genetic sequences and chromosomal structures between different human beings
this is driven by genetic mutations e.g. different physical traits represent genetic diversity in a population

Question 2

What is genetic diversity driven by

Answer 2

Genetic diversity is driven by genetic mutations

Question 3

What is the human genome composed of

Answer 3

23 pairs of homologous chromosomes
22 pairs of autosomes
1 pair of sex chromosomes (XX vs XY)

Question 4

How many base pairs approx in human genomes and what percentage is non-coding

Answer 4

3 billion base pairs, 98% non-coding

Other 2% is made up of around 21,000 genes

Question 5

Most genetic mutations occur in areas where they have ______ effect on life

Answer 5

Most genetic mutations occur in areas where they have little/no effect on life

Question 6

Define allele

Answer 6

1 of 2 alternative forms of a gene present in a chromosome
or
Alternative versions of the same gene

(Aka allelomorph)

Question 7

Factors that affect allele frequencies in a population include

Answer 7

Population size
Mutation rate
Genetic drift (random- affecting small isolated populations)
Natural selection
Environment
Migration
Non-random mating

Question 8

define variants

Answer 8

Variants: DNA changes that occur in areas that do not usually affect human health (but can do)

Question 9

define polymorphism

Answer 9

occurrence of a chromosome/genetic character in more than 1 form, resulting in coexistence of more than 1 morphological type in same population

Variations in population caused by differing alleles (e.g. redheads within a thousand ppl)

Question 10

define mendelian/monogenic inheritance

Answer 10

A disease or trait determined by a single gene (e.g. CFTR - cystic fibrosis is a monogenic recessive disorder)

Question 11

define multifactorial inheritance

Answer 11

A disease or trait determined by a small number of genes (called oligogenic trait), or a large number of genes with individually small effects (polygenic traits). May be ‘triggered’ by environmental factors (ex. Hirschprung disease affects ability to pass faeces).

Question 12

define complex inheritance

Answer 12

A disease or trait determined by a large number of genes and their effects is also altered by a number of environmental factors (such as diet, stress, smoking, etc) (ex. diabetes, cancer, coronary heart disease)

Question 13

Describe homozygous alleles

Answer 13

If both alleles at a specific position on a gene are identical on the chromosomes (one from the mother and one from the father).

Question 14

Describe heterozygous alleles

Answer 14

If the two alleles at a specific position on a gene are not identical

Question 15

define dominant allele

Answer 15

Dominant allele – a gene is in the heterozygous state and one allele is preferentially expressed over the other allele.
It will determine the phenotype both in the homozygous state (2 dominant alleles) and the heterozygous state.

Question 16

define recessive allele

Answer 16

Recessive allele – the allele which does not affect the phenotype in the heterozygous state but will only influence the phenotype in the homozygous state (called homozygous recessive when 2 recessive alleles only are present)

Question 17

what are autosomal chromosomes

Answer 17

Chromosomes that are not directly involved in determining sex -
chromosomes 1-22

Question 18

modes of inheritance in mendelian traits depend on:

Answer 18

Whether the gene is on an autosomal or sex chromosomes

Whether the trait is determined by a dominant or recessive genotype

Question 19

what are the 5 main types of modes of inheritance

Answer 19

Autosomal dominant
Autosomal recessive
X-linked dominant
X-linked recessive
Y-linked

Question 20

what is a dominant allele in simple terms

Answer 20

one copy is enough to bring about disease
If the father is affected then male children are spared; if mother is affected then she can pass trait to both boys and girls

Question 21

Penetrance

Answer 21

Penetrance is
proportion of individuals carry a disease-causing allele

Question 22

Expressivity

Answer 22

Expressivity - extent to which a genotype exhibits its phenotypic expression

Question 23

variable penetrance

Answer 23

Variable penetrance: genetic background, environment; an affected individual MAY be asymptomatic

Question 24

Variable expressivity

Answer 24

Variable expressivity: variation in severity of symptoms
i.e. 1 genotype produces a range of phenotypes

Question 25

Anticipation in autosomal dominant inheritance meaning

Answer 25

Anticipation: disease manifests at a younger age in successive generations (e.g. Huntington’s disease, triplet repeat expansion)
i.e. disease gets worse as generations go on it mutates more

Question 26

explain what is meant by complete penetrance

Answer 26

complete penetrance: if all individuals who have the disease-causing mutation have clinical symptoms of the disease

Question 27

explain what is meant by incomplete penetrance

Answer 27

incomplete penetrance= when we know genotype is present but the phenotype is not observable

Question 28

Define autosomal dominant

Answer 28

autosomal dominant= gene is on a non-sex chromosome, only need 1 copy of mutated allele to cause disorder/characteristic

Question 29

Define autosomal recessive

Answer 29

autosomal recessive= gene is on a non-sex chromosome, need 2 copies of mutated allele to cause disorder/characteristic

Question 30

define x-inactivation in simple terms

Answer 30

early in embryonic development
1 of 2 X chromosomes is randomly + permanently inactivated in cells other than egg cells

Question 31

define X-linked dominant mode of inheritance

Answer 31

trait is on X chromosome
if father is affected, male children r spared
if mother affected, she passes trait to boys + girls

Question 32

define X-linked recessive mode of inheritance

Answer 32

expressed in females if there’s 2 copies of mutated allele.
expressed in males if there’s 1 copy of mutated X-linked recessive allele (as man has 1 X)

Question 33

what is Y-linked inheritance what do we call an affected individual

Answer 33

y-linked inheritance is only father to son transmission

y chromosomes contain few genes (only 200 + most code for male sexual characteristics) so are VERY RARE

affected individuals are called HEMIZYGOTES

Question 34

define hemizygote

Answer 34

hemizygote= only 1 allele is required for disease

Question 35

what is a consanguineous marriage

Answer 35

consanguineous marriage - a union between two individuals who are related as second cousins or closer

Question 36

What is a SNP

Answer 36

Single nucleotide polymorphism= is a variation at a single position (A,C,G,T) in a DNA sequence among individuals