A Level Eduqas Biology: Component 2: "Inheritance"

Define gene

A sequence of DNA that codes for a partuclar polypeptide.

Define allele

Different versions of the same gene e.g. hair colour, in the same gene loci.

Define homozygous

If both alleles are the same

Define filial 1

The first filial generation

Define filial 2

The second filial generation

In what case might a single recessive gene still be shown in the phenotype?

In sex-linked inheritance in a male.

In what case would we reject the null hypothesis?

When our chi squared value exceeds critical value.

What values would we use for degrees of freedom and p in a chi squared calculation?

DOF- number of phenotypes - 1

p- 0.05 unless specified in the question

What are epigenetics?

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

Define dominant

allele will always be expressed

Define recessive

This allele must be present in a pair for it to be expressed in the phenotype

Define codominance

Both alleles of the gene are expressed equally so both characteristics are seen

Define incomplete dominance

Heterozygous individuals produce a phenotype that is an intermediate to the parental phenotypes

What type of linkage is the mendelian cross?

Complete linkage

Define locus

The particular region that the gene occupies on a chromosome

Define genotype

The genetic makeuo of an individual i.e all the alleles that they posess

Define phenotype

The observable characteristics of an organism determined by the genotype

Define locus

The particular region that the gene occupies on a chromosome

Define heterozygous

When both alleles are different

Define monohybrid

The inheritance of a single gene

Define dibybrid

The inheritance of two separate genes

State the 3 characteristics of genes

Genes can mutate

Alleles can seperate and combine during sexual reproduction

A gene is a sequence of codons that specifies the sequence of amino acids in a polypeptide chain

Name the three types of allele combination

Heterozygous, homozygous dominant, homozygous recessive

What is the gene pool?

All the alleles in a population at any one time

Why did Mendel choose peas to investigate?

Peas are easy to grow, can self and cross fertilise, produce flower and fruit in the same year and have characteristics determined by one gene

What characteristics did Mendel choose for his peas?

Tall or dwarf plants, round or wrinkled, yellow and green

State Mendel's first law of inheritance

-Characteristics are controlled by a pair of factors

-Only one factor in gamete

On which type of variation do environmental factors have an effect?

Continuous variation

What type of curve do the values of continous variation have?

Bell shaped

Give some examples of features of continous variation

Height, hair colour, skin colour

Give some examples of features of discontinuous variation

Tongue rolling, blood type

What is the backcross method used to test?

To determine whether a particular dominant characteristic observed in an organism is controlled by one or two dominant alleles

In the backcross test, what is the unknown always crossed with?

-homozygous recessive

What is dihybrid inheritance?

The inheritance of two unlinked genes (genes found on seperate chromosomes).

In dihybrid inheritance, what produces the recombinants or different gametes?

Incomplete assortment

What is the ratio for the F2 cross between heterozygotes in a dihybrid cross?

9:3:3:1

What is Mendel's second law/ law of independent assortment?

Either allele from a pair of alleles may combine randomly with either allele from a second pair of alleles.

Why might the actual ratios of offspring be different from the expected ratio from the genetic cross?

Mutation, environmental factors, linked genes

What is the null hypothesis?

There is no significant difference between the observed and the expected, so any difference is up to chance

What is the alternative hypothesis?

Any difference is due to an external factor that has influence

If our chi squared value exceeds the critical value, what does this mean?

We reject the null hypothesis since other factors are influencing

What is the equation for chi squared?

the sum of: (observed - expected)^(2) ÷ expected

What are autosomes?

Any chromosome that is not a sex chromosome

What is the difference in sex chromosomes in human males and females?

Male: dissimilar

Female: similar

What is the name of alleles carried on the X-chromosomes?

sex-linked chromosomes

What is the consequence of the Y chromosome being much smaller than the X?

Carries far fewer genes/ has less gene loci.

If a male has a sex linked disease, can their son have the disease?

No they cant since all males obtain their X chromosome from their mother and the sex linked disease allele is on the X chromosome of the father

When do females show the phenotype for a sex linked disease?

When both their X chromosomes carry the sex linked reccessive allele

What is haemophilia?

When an individual cant produce enough of one particular blood clotting protein

What is muscular dystrophy?

When an individual doesnt have stable cell membranes of muscle fibres so they lose muscle mass

Why might there be a low number of children who have inherited sex linked diseases?

The affected parents might die before they reproduce or decide they dont wish to concieve.

What does linkage mean?

The tendency of genes that are located close to each other on a chromosome to be inherited together during meiosis and appear in the same gamete

Define incomplete linkage

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

Why might a small number of phenotypes be produced that are different from the gametes that should be produced by linkage?

Crossing over occurs since the genes were further apart on the chromosome. It's rare therefore a small number of gametes are produced with these different genotypes (and thus phenotypes).

Define mutation

-change in the amount,arrangement or structure of DNA

What are the two types of mutation?

Gene mutations: affect individual genes

Chromosome mutations: cause changes in the structure or number of whole chromosome

What are carcinogens?

mutagens that cause cancer

When do mutations most often occur?

Crossing over in Prophase 1

Nondisjunction during Anaphase I and II

Why are gene mutations not as dire as chromosome mutations?

The genetic code is degenerate therefore several codons still code for the same amino acid.

What are mutagens?

Factors in the environment which increase the frequency of mutation in an organism

Give some examples of mutagens

X ray radiation, UV radiation, gamma radiation, heavy metals, cigarette smoke

Which cells, when affected by mutagens, lead to cancer and how?

Protooncogenes and tumour-suppressor genes

They become involved in uncontrollable cell division

What components of tobacco affect the human health?

Tar, carbon monoxide, nicotine

Why does smoking lead to lung cancer specifically?

Tar in tobacco contains carcinogens which target DNA in the cells of alveoli, some of which could become oncogenes.

What could occur if a single base is changed in a gene by mutation?

The DNA base sequence changes, therefore the codons change and code for different amino acids. A different primary polypeptide forms so the bonds between R groups form in different places in the 4° structure resulting in differently shaped proteins

A gene mutation causes sickle cell anaemia. What type of gene mutation is this?

Substitution gene mutation

What causes sickle cell anaemia?

A replacement in one of the base pairs in haemoglobin'a DNA changes an amino acid in two of the polypeptide chains, causing the red blood cells to become sickle shaped.

What is the affect of sickle cell anaemia on the body?

The haemoglobin's affinity for oxygen decreases so it results in anaemia and could be fatal

Sickle cell anaemia is an example of what type of dominance?

Codominant

What word would we use to describe the phenotype for someone who has both a normal haemoglobin allele and a sickle cell allele?

Trait

What are the characteristics of someone who has one sickle cell allele and one normal Hb allele?

Half the cells are sickled so the symptoms of anaemia is milder since oxygen can still be carried. They would be fatigued.

In what cases could sickle cell trait be a selective advantage?

Plasmodium cant successfully reproduce due to the shape of the red blood cell and the short life span of the red blood cells

When do changes in chromosome structure occur due to mutation?

When chromosomes exchange sections of DNA during crossing over at prophase 1

Polyploidy is an example of chromosome mutation; how does it happen?

A gamete recieves two sets of chromosomes during a failed meiotic division so the gametes become diploid instead of haploid.

In which species can a triploid zygote still come to term?

Flowering plants like tomatoes and wheat

What happens in polyploidy occurs in animals?

A triploid or tetraploid zygote would form, which either miscarries or is still born

What is nondisjunction?

A process in which faulty cell division results in one daughter cell getting two copies of a chromosome whilst one daughter cell gets none

Give an example of nondisjunction

Down's syndrome with chromosome 21

Give some features of Down's syndrome?

Small shaped ears, open facial features, height

How does down's syndrome occur?

The oocyte with two copies of 21 chromosome survives and undergoes fertilisation, forming a zygote with 3 21 chromosomes (trisomy 21)

What modifications of the histone proteins could affect the way genes are transcribed in protein synthesis?

DNA can become more tightly coiled around the histone making it harder to unwind and prevents gene expression.

If its could more loosely, it can increase gene expression

How can one gene be expressed differently in one organism?

Epigenetic modifications can occur in cells of the same tissue and in tissues of the same organism

At which point of the cell cycle can the ability for a gene to be transcribed be affected?

After DNA replication

How can methyl groups affect the ability for a gene to be transcribed?

Methyl groups added to nitrogenous bases prevents said bases to be recognised so the whole gene cant be expressed

Define homologous chromosomes

-A pair of chromosomes that have the same genes at each loci

What is sex linkage?

-Gene carried on sex chromosome

What would a conclusion be if the null hypothesis is accepted (calculated value less than critical value)

-The calculated value of _ is smaller than the critical value of _ at p=0.05

-The null hypothesis is therefore accepted

-There is a greater than 5% likelihood that any difference in observed and expected frequencies was due to chance

• Therefore a result of Mendelian patterns of inheritance

What is a test cross used for?

• If a dominant characteristic is determine by one or two dominant alleles.