C23 - Patterns of Inheritance

Question 1

What’s a genotype?

Answer 1

Genetic composition of an organism, which describes all the alleles it contains.

Genotypes for a particular locus can be heterozygous or homozygous.

Question 2

What’s a phenotype?

Answer 2

An organisms observable characteristics. It results from the interaction of genotype and environment.

Question 3

What’s a gene?

Answer 3

A length/section of DNA that codes for the production of a particular polypeptide.

Question 4

What’s a locus?

Answer 4

The position in a chromosome of a particular gene

Question 5

What’s an allele?

Answer 5

A gene variant. A gene can have many different variants and they all occupy the same locus.

Question 6

What does dominant and recessive mean?

Answer 6

Dominant - allele is expressed and affects an organisms phenotype, even when with a recessive allele.

Recessive - allele only expressed when the dominant is absent.

Question 7

What’s codominance?

Answer 7

When two alleles both contribute to the organism’s phenotype to an equal extent.

When 2 codominant alleles are present, both are transcribed and translated to produce polypeptides.

Question 8

What’s cystic fibrosis?

Answer 8

A genetic disease caused by (2) recessive alleles.

Homozygous recessive genotype leads to a faulty transport protein, producing sticky mucus, infertility, poor growth and salty skin.

Normally, the CTFR mRNA is translated to produce a transmembrane protein controlling the movement of Cl- ions. In CF, this protein doesn’t function. (Affected gene locus is on chromosome 7).

Question 9

What are the 3 types of mutation?

Answer 9

Substitution / point mutation - a nucleotide is exchanged for a different one.

Insertion - an extra nucleotide or nucleotides are placed into the DNA sequence.

Deletion - a nucleotide or nucleotides are removed

Question 10

What’s a frameshift?

Answer 10

When insertion or deletion mutations alter all subsequent triplet codes in the DNA sequence, and not only the triplet where the mutation occurs.

This usually produces a non-functioning polypeptide.

Question 11

What are the types of substitution mutation?

Answer 11

Silent - the same amino acid is still coded for due to the code being degenerate

Nonsense - stop codon produced

Missense - a different amino acid is coded for

Question 12

What’s phenylketonuria (PKU)?

Answer 12

A genetic disease caused by inheriting 2 recessive alleles. (Affected gene locus is on chromosome 12).

It prevents the enzyme phenylalanine from being made which can accumulate in babies and cause brain damage.

Question 13

What’s Huntington’s disease?

Answer 13

A genetic disease caused by an insertion mutation in a gene on chromosome 4 / an accumulation of protein fragments in brain neurones.

The (CAG) triplet is repeated over 40 times in the gene
It’s a result of a dominant allele.

Question 14

What’s sickle cell anaemia?

Answer 14

A genetic disease as a result of a substitution mutation in the gene coding for beta polypeptides in haemoglobin.

(Affected gene locus is on chromosome 11).
This mutation causes valine to be added to the polypeptide’s primary sequence instead of glutamic acid.

Question 15

What happens in a silent mutation?

Answer 15

A base in a DNA triplet is substituted for another, however the same amino acid is produced.

Question 16

What happens a nonsense mutation?

Answer 16

A base in a DNA triplet is substituted for another, which results in a stop codon being produced (on the mRNA).

Question 17

What happens in a missense mutation?

Answer 17

A base in a DNA triplet is substituted for another, which codes for a different amino acid.

Question 18

What chromosome locus is affected by cystic fibrosis?

Answer 18

Chromosome 7

Question 19

What chromosome locus is affected by phenylketonuria (PKU)?

Answer 19

Chromosome 12

Question 20

What chromosome locus is affected by Huntington’s disease?

Answer 20

Chromosome 4

Question 21

What chromosome locus is affected by sickle cell anaemia?

Answer 21

Chromosome 11

Question 22

What are the (2) examples of inheritance involving codominance and multiple alleles?

Answer 22

Blood groups

HLA antigens

Question 23

How do blood groups show codominance?

Answer 23

Blood groups A and B show codominance (hence the blood group AB).
The third allele, O, is recessive to A and B.

(IᴬIᴮheterozygotes produce both A and B antigen glycoproteins in erythrocytes plasma membranes).

Question 24

What are HLA antigens?

Answer 24

Human leucocyte antigens, carried on all cells’ plasma membranes.

(These are important when considering organ/tissue transplants to avoid rejection).

They’re the human version of the major histocompatability complex (MHC). Humans have 3 main class 1 MHC genes: HLA-A, HLA-B, HLA-C, and three minor: HLA-E, HLA-F, HLA-G.

Question 25

How do HLA antigens show codominance?

Answer 25

Humans have 3 main class-1 MHC genes: HLA-A, HLA-B, HLA-C, and three minor class-1 MHC genes: HLA-E, HLA-F, HLA-G. The HLA genes (have many variations and) show codominance. All 6 are positioned close together on chromosome 6, meaning they tend to be inherited together.

Question 26

Where are HLA antigens situated (gene locus)?

Answer 26

All 6 are positioned close together on chromosome 6, meaning they tend to be inherited together. This is referred to as a haplotype.

Question 27

What’s a haplotype?

Answer 27

A group of genes within an organism which were inherited together from a single parent.

Question 28

What are the 2 types of genetic linkage?

Answer 28

Sex linkage Autosomal linkage

Question 29

What is Haemophilia A?

Answer 29

A sex-linked condition / an X-linked trait. People with haemophilia suffer internal bleeding and their blood clots more slowly.

Question 30

How causes haemophilia A?

Answer 30

It’s an X-linked trait The allele that produces a working copy (of factor VIII) is dominant (Xᴴ). A mutated variant expresses an altered protein that doesn’t function, which is recessive (Xʰ). If females inherit 2 recessive alleles, this produces a homozygous recessive genotype, resulting in haemophilia (and males only require one as the Y chromosome is unrelated to the condition).

Question 31

What’s an autosome?

Answer 31

Any chromosome that isn’t a sex chromosome.

Question 32

What are the examples of autosomal linkage in inheritance?

Answer 32

ABO blood groups and nail patella syndrome (Nail patella syndrome leads to underdeveloped kneecaps and nails and affects men and women equally).

Question 33

How do ABO blood groups and nail patella syndrome show autosomal linkage?

Answer 33

The allele that causes nail patella syndrome is dominant and is given the letter N. The gene locus is found on chromosome 9, very close to the ABO blood group gene. This proximity means that blood-group and nail patella syndrome alleles will probably be inherited together because crossing over during meiosis is unlikely to separate them.

Question 34

What is a model organism?

Answer 34

A species that is studied to help us understand biological processes and make discoveries that will have relevance to other organisms including humans. (E.g. daphnia or fruit flies (drosophila melanogaster)).

Question 35

What characteristics make [fruit flies / drosophila] a good model organism for genetic research?

Answer 35

A short generation time They breed in large numbers They are small in size and have low cost of care so are easy to breed in lab conditions Scientists have details knowledge of the fruit flies biology and genetics They have several morphological traits e.g. eye colour that have a range of gene variants and are easy to identify

Question 36

What does a dihybrid cross show?

Answer 36

The pattern of inheritance for two traits at different gene loci. E.g. Breeding two individuals who are heterozygous for two genes produces offspring phenotypes in a 9:3:3:1 ratio.

Question 37

How is the Chi-squared test used (with relation to inheritance)?

Answer 37

To analyse whether an inheritance pattern is statistically significant / whether there’s a significant difference between observed and expected numbers of offspring phenotypes.

Question 38

How is a chi squared test carried out (for patterns of inheritance)?

Answer 38

X² = Σ [(O-E)² / E ] X² = the sum of (observed numbers - expected numbers)² / expected numbers This is done using a dihybrid cross [ Columns to calculate: observed O, expected E, O-E, (O-E)², (O-E)²/E ]

Question 39

What’s translocation (in humans)?

Answer 39

When a piece of chromosome breaks off and is transferred to another chromosome. (E.g. Down’s syndrome can be caused by translocation when the end of the long arm of chromosome 21 breaks off and joins another)

Question 40

What’s non-disjunction?

Answer 40

When homologous chromosomes fail to separate in meiosis I, or sister chromatids fail to separate in meiosis II. This results in a gamete that either has a chromosome missing or has an extra chromosome.

Question 41

What’s Down’s syndrome?

Answer 41

A condition caused by a non-disjunction gene mutation, resulting in a gamete with 2 copies of chromosome 21. Therefore, if fertilised, it will have 3 versions of the chromosome / 47 in total.

Question 42

What’s Turner’s syndrome?

Answer 42

A condition caused by a non-disjunction gene mutation where a gamete without a sex chromosome is produced. This results in an embryo with a single X chromosome. (Symptoms include short stature, learning disabilities and obesity).

Question 43

What’s Klinefelter’s syndrome?

Answer 43

``` A condition (only in males) caused by a non-disjunction gene mutation during sperm production, resulting in the oocyte being fertilised with a sperm cell carrying X and Y. This produces an XXY genotype. ``` (Symptoms include infertility, weak bones and less body hair)

Question 44

What are the 3 methods of genetic screening?

Answer 44

Parental carrier testing Prenatal testing Newborn screening

Question 45

What’s parental carrier testing?

Answer 45

When 2 people planning on having a child are tested for the presence of recessive alleles associated with diseases. The couples’s risk of having a child with a genetic condition is then assessed. (Can be used to find the probability of a child having cystic fibrosis).

Question 46

What’s prenatal testing?

Answer 46

When embryos are tested for chromosome mutations and heritable genetic diseases, especially if there’s a high risk of the baby being born with the abnormality.

Question 47

What’s newborn screening?

Answer 47

When babies are tested shortly after birth to identify genetic disorders requiring treatment, e.g. PKU.

Question 48

What’s pedigree analysis?

Answer 48

A method for predicting likelihood of a child being born with a genetic disorder, similar to a family tree.

Question 49

Why (and to who) is genetic counselling offered?

Answer 49

It’s offered to parents when undergoing testing or pedigree analysis, if there’s a known genetic condition in the family or a history of cancer in the family. They help people understand the risks of children inheriting a particular genetic disorder.