Biology 4.3 - Inheritance

Gene - a sequence of DNA that codes for a particular polypeptide and which occupies a specific locus on a chromosome. 

Locus - the position of a gene or mutation on a chromosome. 

Allele - a variant nucleotide sequence of a gene at a given locus, which codes for an altered phenotype.

Dominant - an allele is described as dominant when it is always expressed when present. 

Recessive - an allele is described as recessive when it is not expressed in the presence of a dominant allele. It is only expressed when it is homozygous. 

Phenotype - the appearance of a gene. 

Genotype - the set of alleles that determines a certain trait/characteristic. 

Autosome - any chromosome that is not a sex chromosome. 

Sex chromosome - a chromosome concerned in the determination of the sex of an organism.

Monohybrid inheritance = the inheritance of a single gene. 

Gregor Mendel investigated inheritance in peas. This was a useful choice because:

• They are easy to grow.
• Their flowers can self fertilise AND cross fertilise. 
• They produce flowers and fruit in the same year.
• Each cross produces a large number of seeds.

Mendel investigated contrasting characteristics:

• Tall and dwarf plants.
• Round and wrinkled seeds.
• Green and yellow seeds. 

These characteristics are:

• Controlled by single genes.
• The genes are found on different chromosomes.
• Clear cut and easy to distinguish.

Monohybrid crosses show:

• The generations.
• The genotypes of the parents and offspring. 
• The phenotypes of the parents and offspring. 
• The gametes.
• The symbols for the alleles are defined. 

A test cross is used when you have an individual with a phenotype of a dominant characteristic but an unknown genotype (could be homozygous dominant or heterozygous). Therefore, a test cross is performed in which the individual breeds with an organism that has a recessive characteristic: they must be homozygous recessive.

For example:

If it is known that the allele producing tall plants is dominant (T), and we are crossing a tall plant with an unknown genotype against a dwarf plant (tt), then:

• If 100% of the offspring are Tt, then the genotype of the dominant individual must have been TT.
• If 50% of the offspring are Tt and 50% are tt, then the genotype of the dominant parent must have been Tt. 

When genes are co-dominant, then the alleles of the heterozygous pair are both expressed individually. This means the phenotype is a combination of both of the alleles' characteristics. 

E.g. Chickens with black feathers (BB) and chickens with white feathers (bb) can breed to produce speckled chickens (Bb).

Incomplete dominance is when the phenotype for a heterozygous pair is an intermediate between the parental phenotypes. 

E.g. Red carnations (RR) and white carnations (rr) will breed to produce pink carnations (Rr).

NOTE: if offspring have phenotypes in the ratio 1:2:1, it is likely their parents are heterozygous for a gene that shows either co-dominance or incomplete dominance.

From this, Mendel's first law of inheritance, the law of segregation is established. This states that the characteristics of an individual are determined alleles that occur in pairs. In gametes, only one of the pair is present. 

Dihybrid inheritance is the simultaneous inheritance of two unlinked genes (e.g. the genes are found on different chromosomes).

Dihybrid ratio: if two individuals are heterozygous for both genes they will produce offspring with the phenotypic ratio of 9:3:3:1. 

From the formulation of this ratio, Mendel's second law of inheritance can be established: the law of independent assortment. This states that allele pairs will separate randomly and independently during the formation of gametes.

Unlinked genes follow Mendel's law of independent assortment: the alleles will separate independently and randomly during the formation of gametes. 

Unlinked genes are either found on separate chromosomes, OR far apart on the same chromosome so that crossing over during prophase I of meiosis I separates them 50% of the time. 

Independent assortment:

• Occurs during metaphase I of meiosis I.
• Two copies of a gene are found at the same locus on two different chromosomes of a homologous pair. 
• An organism receives two homologous pairs: maternal and paternal.
• These homologous pairs will align on the equator randomly, with either chromosome facing either pole. This is independent assortment. 
• Therefore, either of a pair of alleles can combine with either of another pair of alleles.
• The gametes receive a mixture of alleles. 
• The genes are therefore UNLINKED. 

Linked genes are found on the same chromosome and cannot separate independently at meiosis. 

HOWEVER... 

Crossing over occurs at prophase I of meiosis I, giving rise to the opportunity to produce recombinants. But this depends on how far apart the two genes are on the chromosome: the further apart the genes are, the more opportunity there is for a crossover to occur between them. 

IF CROSSING OVER DOES NOT OCCUR: the products are parental gametes - they have a phenotype identical to the parent chromosome.

IF CROSSING OVER DOES OCCUR: the products are recombinant gametes - they have a different phenotype to the parent chromosome. 

• Crossing over is rare = it does not occur in most cells.
• As a result most gametes are parental. 
• Thus the number of gametes with different genotypes is not equal.
• Most offspring will have parental phenotypes.
• Mendelian ratios are not produced. 

If a set of values does not follow the Mendelian ratio, it is likely the genes involved are linked. 

Equation:

Null hypothesis: there is no difference between the observed and expected values of the cross. 

How do you calculate the expected values?

• Add the observed values and divide by 16.
• Multiply this value by either 9, 3 or 1 (depending on the Mendelian ratio).

Degrees of freedom: (n-1)

If the calculated value of x² is less than the critical value at the 5% significance level:

• This is equivalent to a probability of greater than 5%
• The null hypothesis is accepted at the 5% significant level.
• Inheritance is Mendelian and any deviation from the expected value is due to chance or sampling error. 

If the calculated value of x² is greater than the critical value at the 5% significance level:

• This is equivalent to a probability of less than 5%
• The null hypothesis is rejected at the 5% significance level.
• There must be some other explanation for the date (i.e. the genes are LINKED). 

• Human cells contain 46 chromosomes (23 pairs).
• 23 from each parent. 
• Chromosomes are arranged in homologous pairs. Each pair contains chromosomes of the same size and shape, with genes in the same order, coding for the same characteristics. 
• 22 pairs have identical genes = they are autosomes. 
• The 23 pair is the sex chromosomes.
• ** = female = identical.
• XY = male = the Y chromosome is much shorter. 
• Therefore, these chromosomes are referred to as heterosomes. 

NOTE: the arrangement of homologous chromosomes in decreasing size order is called a karyotype. 

They are useful as they help to determine whether a conditions is:

• Sex-linked.
• A result of dominant or recessive alleles. 

• Caused by an X-linked recessive allele.
• It leads to the gradual loss of muscle mass and progressive muscle weakness. 
• As a result of being X linked, if it is present in males, it is expressed. 

A mutation is a random, spontaenous event that changes the amount, arrangement or structure of DNA.

Mutations can occur in all cells, but only mutations in gametes can be inherited. Mutations can also contribute to variation between individuals, which can result in evolution through natural selection. 

In haploid organisms, any mutation will always be expressed in the phenotype. In diploid organisms, the mutation can either be dominant or recessive. Dominant mutations are always expressed but are rare. The likelihood is that the mutation is recessive and occurs in a cell with a dominant allele. Therefore, it is likely not to be expressed in diploid organisms. So mutation has less of an impact on evolution in diploid organisms. 

Mutations occur during crossing over of prophase I or non-disjunction of anaphase I and II. Mutation rates are normally low. However, in organisms with short life cycles, cell division occurs more frequently. As a result, they show a greater rate of mutation. 

Rates can be increased by:

• Ionising radiation (e.g. gamma rays, x-rays and UV light).
• Mutagenic chemicals (e.g. polycyclic hydrocarbons in cigarette smoke). 

Gene point mutation: occurs when DNA is not copied accurately in the S phase, before cell division. These errors involve one/ a small number of bases.

The sequence of bases can be changed in several ways:

• ADDITION of a new base.
• SUBTRACTION of a base.
• SUBSTITUTION of a new base.
• DUPLICATION of a base.
• INVERSION - two adjacent bases on the same DNA strand can exchange position. 

NOTE: if three new bases are added then an extra amino acid is added to the polypeptide chain, and visa versa. 

Gene point mutations can change the bases of a messenger RNA codon. The effect on the polypeptide produced during translation and consequently the effect on its phenotype depends on the nature of the mutation:

• Some mutations will create codons that code for the same polypeptide = silent mutation.
• An amino acid may be substituted that is similar to the original amino acid = small change. 
• The mutation may have a significant effect on the activity of the protein.

NOTE: if the protein is an enzyme, the structure of the active site can be altered and so the enzymes can be denatured as a result.

An example of a gene point mutation is sickle cell anaemia, which is a result of a substitution point mutation.

Sickle cell anaemia occurs due to a substitution point mutation of the B-polypeptide in haemoglobin.

IN SUMMARY:

The triplet code CTC (amino acid = glutamate) is replaced by CAC (amino acid = valine). As a result the cell membrane collapses and the red blood cells become sickle shaped. They are fragile and can break in the capillaries. Their shape also severly reduces their ability to carry oxygen, resulting in anaemia and possible death.

Sickle cell anaemia shows co-dominance, in which an individual heterozygous for the gene will have 50% normal red blood cells. The other 50% will be sickle shaped.

There are two types:

1. Changes to the structure of chromosomes.

This occurs during crossing over of prophase I of meiosis I. Following crossing over, homologous pairs may not rejoin accurately, resulting in homologous chromosomes (and gametes) with different genes.

2. Changes to the number of chromosomes.

This occurs during non-disjunction of anaphase I and II of meiosis. Non-disjunction is when a faulty cell division results in one of the daughter cells receiving two copies of a chromosome, and the other receiving none. An example of this occurs in Down's syndrome.

• Affects chromosome number 21. 

If non-disjunction occurs during oogenesis, then the secondary oocyte either has:

1. No chromosome 21 - a viable embryo cannot be produced as a result.

2. Two copies of chromosome 21 - when the secondary oocyte fuses with normal sperm a viable embryo is produced with 3 copies of chromosome 21. The individual has 47 chromosomes in total.

Tumour suppressor genes regulate mitosis and ensures cells do not divide too quickly. HOWEVER, a muation in these genes could cause continual, repeated mitosis, creating a large number of cells. If these cells escape the attack of the immune response, they will form a bundle of cells = tumour.

A tumour can be harmless. However, the tumour can also spread around the body and invades other tissues, forming secondary tumours (metastases). This can cause cancer. 

Abnormalities in the tumour suppressor gene TP53, which codes for protein P53, has been identified in over half of all human cancers.

A proto-oncogene is a gene that codes for a protein that contributes to cell division. Therefore, a mutation one of these genes may switch it on permanently, resulting in a large number of proteins being synthesised = RAPID, REPEATED MITOSIS. 

A large number of cells would form (tumour). This can lead to cancer.

Therefore, a mutated proto-oncogene that causes cancer is called an ONCOGENE.