Chapter 16: Inheritance and Genetic Crosses
- Genetics: study of inheritance.
- Inheritance: passing on of traits from one generation to the next.
- Traits: physical/chemical characteristics that a living organism possesses.
- Gamete: haploid sex cell.
- Fertilisation: fusion of two haploid gametes to produce a diploid zygote.
- Allele: alternative form of the same gene where a number of different types of the same gene exist.
- Locus: position of an allele or gene on a chromosome.
- Homozygous: two alleles are the same.
- Heterozygous: two alleles are different.
- Dominance: one allele masks the effects of another allele.
- Recessive: allele’s effect is only expressed in the homozygous condition.
- Genotype: genetic make-up of an individual.
- Phenotype: physical make-up of an individual.
There can be a number of different alleles controlling the same characteristics; e.g. eye colour in humans: blue, green, brown, hazel are some of the common colours. Each organism has a maximum of two alleles for each characteristic; e.g. a person with brown eyes could have two brown eye alleles or one brown eye allele and one blue eye allele. Alleles are usually assigned letters with a capital letter signifying a dominant allele and a lower-case letter signifying a recessive allele. B (brown eyes); b (blue eyes), etc.
- BB is described as homozygous dominant.
- Bb is described as heterozygous.
- bb is described as homozygous recessive.
Genetic cross: diagram or table showing how characteristics are inherited.
Monohybrid cross: genetic mating between two organisms where one gene is studied.
Examples of characteristics that can be studied using monohybrid crosses:
- Ability to tongue roll (dominant) versus inability to tongue roll (recessive).
- Cleft chin (dominant) versus non-cleft chin (recessive).
- Dimples (dominant) versus no dimples (recessive).
- Free ear lobes (dominant) versus attached ear lobes (recessive).
- Long second toe (dominant) versus short second toe (recessive).
- Widow’s peak (dominant) versus no widow’s peak (recessive).
- Straight thumb (dominant) versus curved thumb (recessive).
Study of the inheritance of single traits to the first filial generation involving homozygous parents.
In some species XX are male and XY are female! Examples include: some birds, some reptiles, moths and butterflies.
- Incomplete dominance: neither allele of an allelic pair is dominant or recessive with respect to each other – they are equally expressed and the resulting phenotype is a mixture, or blend, of the two.
An example of incomplete dominance is flower colour in the snapdragon plant. A red-flowered snapdragon plant (RR) crossed with a white-flowered snapdragon plant (rr) produces pink-flowered offspring (see below).
Work of Gregor Mendel
Mendel was an Augustinian monk, known as the father of modern genetics. Mendel carried out genetics studies on pea plants. He studied seven characteristics:
1. Flower colour (purple versus white)
2. Flower position (axial versus terminal)
3. Pea colour (yellow versus green)
4. Pea shape (round versus wrinkled)
5. Pod colour (green versus yellow)
6. Pod shape (inflated versus constricted)
7. Height (tall versus short)
As a result of his work, Mendel came up with his two Laws of Genetics:
- First Law of Segregation:
Each cell contains two factors for each trait. These factors separate at gamete formation, so that each gamete contains only one factor from each pair of factors. At fertilisation, the new organism will have two factors for each trait, one from each parent.
- Second Law of Independent Assortment:
Members of one pair of factors separate independently of another pair of factors during gamete formation.
Explanation of Mendel’s First Law of Segregation: Chromosomes are arranged into homologous pairs. During meiosis, half of the gametes receive one of the homologous chromosomes with the other half of gametes receiving the other homologous chromosome.
Explanation of Mendel’s Second Law of Independent Assortment: Mendel’s second law applies to crosses involving more than one gene, that is, two pairs of alleles. Each allele of a pair can combine completely randomly with either member of another pair (see below).
- Dihybrid cross: genetic mating between two organisms where two separate genes are studied.
Study of the inheritance to the second filial generation of two traits using the Punnett square technique:
In Mendel’s genetic experiments on pea plants, he studied two traits at the same time. He also studied how these traits were inherited through two generations. He found the ratios of the resulting offspring. One pair of traits that he studied at the same time in pea plants was height and flower colour. He took pure-breeding (homozygous for both traits) pea plants and crossed them. This was a cross between homozygous plants. One plant was tall and had purple flowers. The other plant was short and had white flowers. He found that the offspring of a cross between these parents always produced plants that were tall and purple-flowered. This was called the F1 generation.
9 tall and purple-flowered pea plants
3 tall and white-flowered pea plants
3 short and purple-flowered pea plants
1 short and white-flowered pea plant
Another cross Mendel studied was a mating between a tall, purple-flowered pea plant (heterozygous for both traits) and a short, white-flowered pea plant. He discovered that the offspring from this cross had phenotypes that appeared in a 1:1:1:1 ratio.
- Linkage: genes are present on the same chromosome.
Below is the cross Mendel performed on pea plants where the genes are linked. The ratio of the offspring phenotypes changes when genes are linked because linked genes tend to stay together during gamete formation (meiosis). When genes are on the same chromosome (linked) there are fewer unique gametes.
- Sex linkage: genes are located on the X chromosome.
In humans, female are ‘XX’ whereas males possess a ‘y’ chromosome and are therefore ‘XY’.
The ‘X’ chromosome is longer than the ‘Y’ chromosome.
This means that many genes that are present on the ‘X’ chromosome are not present on the ‘Y’chromosome.
Males therefore only have one copy of many sex-linked genes rather than the usual two.
Two examples of sex-linked characteristics in humans that you must learn about are:
- Red-green colour vision
Both of these characteristics are controlled by genes present only on the ‘X’ chromosome. There is no corresponding gene for these characteristics on the ‘Y’ chromosome. This is represented in genetic crosses as ‘Y_‘.
As a result of males having only one copy of these genes they are much more likely to suffer from the corresponding genetic conditions should they inherit an ‘X’ chromosome with a mutated gene from their mother.
The corresponding conditions are:
- Red-green colour blindness
If they inherit a normal gene from their mother, this is represented by ‘XN‘. If they receive a mutated gene from their mother, this is represented as ‘Xn‘.
Pedigree studies are used by geneticists and genetic counsellors for determining and explaining inheritance of certain characteristics. Squares always represent males with circles representing females. Inheritance of cystic fibrosis is often explained using pedigree studies.
Non-nuclear inheritance refers to inheritance of DNA via the mitochondria and chloroplasts. It does not follow Mendel’s Laws of Genetics as mitochondria and chloroplasts are inherited independently of the nucleus and always inherited maternally (i.e. via the egg cell because it is the egg cells that carry the cell organelles).