- The study of inheritance of characteristics at the whole organism level is also known
as classical genetics or Mendelian genetics, since it was pioneered by Gregor Mendel.
- Mendel investigated simple qualitative characteristics (or traits), such as flower colour or seed shape, and he varied one trait at a time.
- Mendel used an organism whose sexual reproduction he could easily control by carefully pollinating stigmas with pollen using a brush.
- Peas can also beself pollinated, allowing self crosses to be performed.
- This is not possible with animals.
- Mendel repeated his crosses hundreds of times and applied statistical tests to his results.
- Mendel studied two generations of peas at a time.
The outward appearance (the phenotype) is not necessarily the same as the inherited factors (the genotype).
Mendelian Genetics 2
One form of a characteristic can mask the other.The two forms are called dominant and recessive respectively.
The F2 ratio is always close to 3:1 (or 75%:25%).
Mendel’s factors are now called genes and we know they are found on chromosomes.
The two alternative forms are called alleles and are found on homologous pairs of chromosomes (the maternal and paternal).
One allele comes from each parent, and the two alleles are found on the same position (or locus) on the homologous chromosomes.
If the homologous chromosomes have the same alleles at a locus this is homozygous, and if they have different alleles this is heterozygous.
The dominant allele is defined as the allele that is expressed in the heterozygous state, while the recessive allele is defined as the allele that is only expressed in the homozygous state (or is not expressed in the heterozygous state).
The Monohybrid Cross
A simple breeding experiment involving just a single characteristic, like Mendel’s experiment, is called a monohybrid cross.
- At fertilisation any male gamete can fertilise any female gamete at random.
- The possible results of a fertilisation can most easily be worked out using a Punnett Square.
- Each of the possible outcomes has an equal chance of happening, so this explains the 3:1 ratio observed by Mendel.
- This is summarised in Mendel’s First Law, which states that individuals carry two discrete hereditary factors (alleles) controlling each characteristic.
- The two alleles segregate (or separate) during meiosis, so each gamete carries only one of the two alleles.
The Test Cross
- If the offspring all show the dominant trait then the parent must be homozygous dominant.
- If the offspring are a mixture of phenotypes in a 1:1 ratio, then the parent must be heterozygous.
How does Genotype control Phenotype?
- The dominant allele is the normal (or “wild-type”) form of the gene that codes for a functioning enzyme.
- The recessive allele is a mutation of the gene.
- This mutated gene codes for non-functional enzyme.
- Almost any mutation in a gene will resultin an inactive gene product (often an enzyme), since there are far more ways of making an inactive protein than a working one.
- Sometimes the gene actually codes for a protein apparently unrelated to the phenotype.
- Sex chromosomes (X andY) are non-homologous they are called heterosomes, while the other 22 pairs are called autosomes.
- In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY), though in other species it is the other way round.
- There will always be a 1:1 ratio o fmales to females.
- Female gametes (eggs) always contain a single X chromosome, while the male gametes (sperm) can contain a single X or a single Y chromosome.
- Sex is therefore determined solely by the sperm.
- There are techniques for separating X and Y sperm, and this is used for planned sex determination in farm animals using artificial insemination (AI).
- In humans it is the Y chromosome that actually determines sex: all embryos start developing as females, but if the sex-determining “SRY” gene on the Y chromosome is expressed, male hormones are produced in the embryo, causing the development of male characteristics.
- In the absence of male hormones, the embryo continues to develop as a female.
- The X chromosome is not involved in sex determination.
- The X chromosome, is large and contains over a thousand genes that have nothing to do with sex, coding for important products such as rhodopsin, blood clotting proteins and muscle proteins.
- Females have two copies of each gene on the X chromosome (i.e. they’re diploid), but males only have one copy of each gene on the X chromosome (i.e. they’re haploid).
- This means that the inheritance of these genes is different for males and females, so they are called sex linked characteristics.
- Males always inherit their X chromosome from their mothers, and always pass on their X chromosome to their daughters.
- In some cases there are three phenotypes, because neither allele is
completely dominant over the other, so the heterozygous genotype has its own phenotype.
- This situation is called codominance or incomplete dominance.
- Since there is no dominance we can no longer use capital and small letters to indicate the alleles, so a more formal system is used. The gene is represented by a letter and the different alleles by superscripts to the gene letter.
Sickle Cell Anaemia
Another example of codominance is sickle cell haemoglobin in humans.
- The gene for haemoglobin (or more accurately for the polypeptide globin – see unit 1) “Hb” has two codominant alleles: HbA (the normal gene) and HbS(the mutated gene).
- The mutation inthe HbSgene is a single base substitution (TA), changing one amino acid out of 146 in the polypeptide chain.
- This amino acid binds to other haemoglobin molecules, so the molecules link together to form long chains, distorting the red blood cells into sickle shapes.
- An individual has two copies of each gene, so can only have two alleles of any gene, but there can be more than two alleles of a gene in a population.
- An example of this is blood group in humans. The red bloodcell antigen is coded for by the gene I(for isohaemaglutinogen), which has three alleles IA, IBand Io.
- (They are written this way to show that they are alleles of the same gene.)
- IA and IBare codominant, while Io is recessive.