4.3 Inheritance

Gene

A sequence of DNA that codes for a polypeptide and which occupies a specific locus on a chromosome

Allele

A variant nucleotide sequence for a particular gene at a given locus, which codes for an altered phenotype

Locus

A specific position on a chromosome where a gene is located

Recessive

A recessive allele will be ‘hidden’ when a dominant allele is present in a heterozygote. A recessive allele will only be expressed when it is homozygous.

Codominant

Alleles that are equally expressed in a heterozygote.

Phenotype

The characteristics of an organism resulting from both its genotype and the effects of the environment.

Genotype

The genetic make-up of an organism i.e. its alleles.

Homozygous

Both alleles for a gene are identical.

Heterozygous

Both alleles for a gene are different.

F₁

The first filial generation - the offspring of the parents in a genetic cross.

Autosomes

Chromosome which are not sex chromosomes

Sex Chromosome/ heterosome

Chromosomes which determine the sex of an individual organism.

Why did Mendel choose peas for his experiment

• They are easy to grow

• Their flowers can self fertilise or cross fertilise

• They make flowers and fruit the same years

• They make a large number of seeds from each cross

Why was it important that Mendel’s peas made a large number of seeds from each cross

Their numbers make them statistically meaningful

How was Mendel fortunate in his choice of characters in pea plants

• Controlled by single genes

• Controlled by genes on different chromosomes

• Clear-cut and easy to tell apart

Describe how Mendel carried out his hybridisations

1) Choses two true breeding individuals that have different traits.

2) Manually transferred pollen from the anther of the mature pea plant of one variety to the stigma of a separate mature pea plant.

3) Mendel waited for the plant to produce seeds which he collected and grew the folllowing year

Practical precautions that should be taken to ensure that offspring produce by plants are only from the desired cross

{Anthers/ stamen} removed (1)

Pollen transferred by hand (1)

(Recipient) flower is isolated/ method of isolation e.g. in a bag (1)

The male and female parts are separated (1)

State the Mendel’s first law of heredity - The law of segregation

The characteristics of an organism are determined by factors (genes) which occur in pairs. Only one

member of a pair of factors (genes) can be represented in a single gamete.

What is a test cross

Cross between an individual with the phenotype of the dominant characteristic, but unknown genotype with an individual that is homozygous recessive for the gene in question

What is monohybrid inheritance

The inheritance of a single gene, such as that controlling plant height or seed colour

Define codominance

When both alleles in a heterozygote are fully and equally expressed in the phenotype. Neither allele masks the other

Define incomplete dominance

neither allele is fully dominant, so the heterozygote has a blended or intermediate phenotype between the 2 homozygotes

Example of codominance

Roan cattle, the heterozygous genotype shows both red and white hairs at the same time

Example of incomplete dominance

Crossing red and white snap dragons to produce pink flowers

Compare the expression of alleles in codominance and incomplete dominance

Codominance = both alleles are equally and fully expressed

Incomplete dominance = Neither allele is fully expressed

What two regions of the human X and Y chromosomes are homologous and can pair with each other at meiosis

Pseudoautosomal regions, PAR1 and PAR2

How are females the homogametic sex

all secondary oocytes contains an X chromosome

How are males the heterogametic sex

At meiosis I an X chromosome passes into one secondary spermatocyte and a Y passes into the other. Half the male sperm contains an X chromosome and half contain a Y

How is there equal chance of the foetus being male or female

At fertilisation the oocyte may be fertilised by either an X carrying sperm of Y carrying sperm with equal probability

Define sex linkage

A gene is carried by a X chromosome so that a characteristic it encodes is seen predominately in one sex

State Mendel’s Second law - The law of independent assortment

Each of a pair of contrasted characters may be combined with either of another pair.

What is dihybrid inheritance

The simultaneous inheritance of two unlinked genes i.e genes on different chromosomes

Define true breeding

If a plant is true-breeding it has been self-fertilised for many generations, always producing the same phenotype. It has a homozygous genotype.

Example of dihybrid inheritance genetic diagrams

When do Mendel’s laws apply

When the genes are on different chromosomes

How is the dihybrid test cross different from the monohybrid test cross

The organism being tested is crossed with one that is recessive for both characteristics

What is the expected ratio if a double heterozygous individual is crossed with an individual homozygous for both characteristics

1:1:1:1

Define linkage

Description of genes that are on the same chromosomes and therefore don’t segregate independently of each other

Why are linked genes inherited together

They move together during meiosis and appear in the same gamete

Where does crossing over occur

between chromatids at the chiasmata

When does recombination take place

when alleles are exchanged between homologous chromosomes as a result of crossing over

What effects the chance of crossing over

The further apart two genes are on a chromosome, the more chance there is of crossing over,

Why is crossing over important

If crossing over occurs, alleles that were previously linked get separated. This leads to recombinant genotypes.

Increases genetic variation

Phenotypic ratio for when a heterozygous heterozygous is crossed with another heterozygous heterozygous

9:3:3:1

Ratio for when RRYY is crossed with rryy

RrYy

All round and yellow

Ratio for when RrYY and rryy

RrYy and Rryy

1:1 round yellow : wrinkled yellow

Ratio for RRYy and rryy

RrYy and Rryy

1:1 round yellow: round green

Ratio for RrYy and rryy

1:1:1:1

Round yellow : Wrinkled yellow : Round green : wrinkled green

Define mutation

A change in the amount, arrangement or structure in the heredity material of an organism, either DNA or in the case of some viruses RNA

How can mutations be described as spontaneous

as they may happen without an apparent cause

How can mutations be described as Random

as they appear to happen with equal probability anywhere in the genome of diploid organisms

What mutations can be inherited

Only mutations that occur in gametes

What organisms show a greater rate of mutations

Organisms with short life cycles and frequent meiosis

Why do organisms with short life spans have a faster rate of mutation

Because mutation occurs during DNA replication and during cell division

How can mutation rates be increased

• Ionising radiation

• Mutagenic chemicals

How can mutations occur

• Gene or point mutation

• Chromosome mutation

• Aneuploidy

• Polyploidy

What is Aneuploidy

A whole chromosome or a small number of chromosomes may be lost or added, in a phenomenon called non disjunction

What is non disjunction

When chromosomes fail to separate at the poles of dividing cells at anaphase I or when chromatids fail to separate at anaphase II

What is polyploidy

The number of chromosomes may double following the first nuclear division after fertilisation

When does a gene mutation occur

If the DNA polymerase changes the base sequence

Gene mutation addition

A base is added

Gene mutation duplication

The same base is incorporated twice

Gene mutation subtraction

A base is deleted

Gene mutation substitution

a different base is incorporated

Gene mutation inversion

adjacent bases on the same DNA strand exchange position

What is a chromosome mutation

changes in the structure or number of chromosomes in cells

When does mutation arise in chromosomes

When a chromosome does not re-join accurately at the corresponding position on its homologous partner after crossing over in prophase I

What does a faulty spindle result in

The chromosomes not being shared equally between the daughter cells. One of the daughter cells receive two copies of a chromosome while the other gets none.

What are cells with complete sets of chromosomes described as

euploid

Name for cells with a small number of extra chromosomes or a small number too few?

aneuploid

Why might polyploidy arise

• If two diploid gametes fuse

• Endomitosis

• A defect in the spindle

Why is polyploidy more common in plants

• They can reproduce asexually

• They are hermaphroditic and so do not use chromosomes to determine their sex

How can damaged chromosomes result in mutations

If chromosomes break they will normally repair themselves but they may not repair themselves correctly.

What is sickle cell anaemia

A mutation in the DNA coding for the beta globin chain of the haemoglobin molecule

What is a genotype of a sufferer from sickle cell

HbS HbS

What condition do heterogenous sickle cell individuals have

The sickle cell trait HbS HbA

Where does the substitution occur on the beta chains in sickle cell

Position 6

What does the base substitution in Sickle cell anaemia cause

Glutamic acid is replaced by valine

How is the solubility of the haemoglobin effected by sickle cell

1. A hydrophilic molecule (glutamic acid) is switched to a hydrophobic molecule (valine)

2. Position 6 is located at the surface of the beta chain where it would normally be exposed to water.

3. The switch in amino acids reduces the solubility and promotes the formation of large insoluble aggregates

What base is substituted in sickle cell

The substitution from T to A

Why has the mutation for sickle cell survived in populations in areas of the world where malaria is

The carriers of sickle cell are relatively protected against malaria

How are the carriers of sickle cell protected against malaria

• Life cycle of the parasite involves cycles of multiplication in red blood cells.

• The parasite has problems surviving in hosts whose red blood cells are compromised by the presence of HbS

• Carries of the sickle cell gene who have just one copy of the gene survive the infection.

What produces an individual with Down’s syndrome

1. Non-disjunction happens during oogenesis.

2. Homologous chromosome fail to separate

3. Secondary oocyte either has no chromosome 21 or has two copies.

4. The secondary oocyte fuses with a normal sperm producing a viable embryo with cells containing 3 copies of chromosome 21

Another name for Down’s syndrome

trisomy 21

How can Down’s syndrome be hereditary

Translocation Down’s

Describe translocation Down’s

1. During meiosis a fragment of chromosome 21 attached itself to chromosome 14.

2. When that abnormal gamete fused with a normal one it produces an embryo with two normal copies of chromosome 21 and the fragment of the additional one attached to chromosome 14.

What happens if an organism has an uneven number of chromosomes

bivalents cannot form during prophase I

Meiosis does not continue

Gametes are not produced

What is a carcinogen

An agent which causes cancer

Name of genes that regulate mitosis and prevent cells dividing too quickly

Tumour suppressor cells

How might a tumour suppressor gene lose its regulatory function

a mutation occurs

What is a proto oncogene

Codes for a protein that contributes to cell division

What is a oncogene

A proto oncogene with a mutation that results in cancer

How may a proto oncogene be mutated

A mutation causes chromosomes to rearrange and places the proto oncogene next to a DNA sequence that permanently activates it

There is an extra copy of the proto oncogene resulting in too much of its product being made, causing excessive mitosis

What is the variation within a species related to

• Differences in DNA nucleotide sequence

• Physiological effects of the environment

Define epigenetic

The control of gene expression by modifying DNA or histones but not by affecting the DNA nucleotide sequence

What groups can cytosine have

A methyl or hydroxymethyl group

What is the effect of regions of DNA being heavily methylated

They are less likely to be transcribed and it is less likely that the gene will be expressed

What are promoter regions

Sequence of DNA before the first exon