Genetic Inheritance & Genetic Crossing (SL IB Biology)

• Gametes are the sex cells of an organism
• For example, the sperm and egg (ovum) cells in humans
  • The egg is larger than the sperm as most of its space contains food to nourish a growing embryo
  • The sperm cell contains many mitochondria to release energy for its motion
• Gametes fuse during fertilization to form a zygote (fertilised egg cell)
• These sex cells are formed during meiosis and only have one copy of each chromosome and so are haploid cells
  • For humans, that means the sperm and egg cells contain 23 single chromosomes in their nucleus (as opposed to diploid cells which contain 46 chromosomes, or 23 pairs)
  • As there is only one chromosome from each homologous pair there is only one allele of each gene present
    • This allele may be dominant, recessive or co-dominant

Sperm cell diagram

Egg cell diagram

The structure of human gametes - the sperm and egg

• Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles
• Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
• Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
• When a male and female gamete fuse their chromosomes are combined
• This means the resulting zygote is diploid
  • The zygote contains two chromosomes of each type
• It will therefore have two alleles of each gene
  • If the two alleles for a particular gene are the same then the genotype is described as homozygous
  • If the two alleles for a particular gene are different then the genotype is described as heterozygous

• Gregor Mendel was an Austrian monk
• In the mid-19th century, Mendel carried out breeding experiments on large numbers of pea plants whilst looking after the monastery gardens
• He studied how characteristics were passed on between generations of plants
• Due to his extensive work on the understanding of inheritance, he is sometimes called the Father of Genetics 
• Mendel carefully transferred pollen from one pea plant to the reproductive parts of another
  • Pollen contains the male gamete and is located on the anther of the flower
  • The female gametes are located in the ovary
  • The plants reproduce sexually and require pollination for fertilisation
  • This technique eliminated any uncertainty from his data since he knew which pollen had fertilised each of the plants 
• He collected the pea seeds from these plants and grew them in favourable conditions to find out their characteristics
• He also cross-bred offspring peas in order to find out which, if any characteristics would appear in future generations
• Mendel investigated the height of pea plants, the colours of their flowers and the smoothness of their seed coat

Mendel's breeding experiments of pea plants diagram

Mendel's pea plant crosses

Mendel's Pea Plant Results Table

• Mendel found that characteristics were inherited in a predictable pattern
• All pea plants in the first generation had the same characteristic as one of the parental plants
• The offspring plants in the second generation had characteristics of both parent plants in a 3:1 ratio
• Without knowing it, Mendel had discovered genes, he referred to them as 'units of inheritance'
• He also discovered that some genes are dominant and some genes are recessive
• Different forms of the same gene are called alleles

• A monohybrid trait is one that is controlled by only one gene
• Generally, we consider that such a gene has two alleles
  • Either: one allele is dominant and the other is recessive
  • Or: the alleles are co-dominant
• A monohybrid cross starts with pure-breeding parents (homozygous), each displaying a different phenotype
  • This generation is known as the parental generation, denoted as the P generation
• The purpose of a Punnett grid is to predict the probability of a certain offspring displaying a certain genotype or phenotype
  • In the case where multiple offspring are produced, Punnett grids can predict the numbers of offspring that will display a certain genotype or phenotype after a cross

Steps in constructing a Punnett Grid

1. Write down the parental phenotypes and genotypes
2. Write down all the possible gamete genotypes that each parent could produce for sexual reproduction
   • A useful convention is to write the gamete genotypes inside a circle to denote them as gametes (haploid cells)
3. Place each parental genotype against one axis of a Punnett grid (2 x 2 table)
4. In the boxes of the Punnett grid, combine the gametes into the possible genotypes of the offspring
   • This gives the offspring of the F₁ generation (1^st filial generation)
5. List the phenotype and genotype ratios for the offspring

Step 1: Write down the parental phenotype and genotypes

Green coloured pods                            Yellow coloured pods

GG                                                    gg

Step 2: Write down all the possible gamete genotypes that each parent could produce

Step 3: Place each parental genotype against one axis of a Punnett grid (2 x 2 table)

Step 4: Combine the gametes in each box of the Punnett grid

Genotypes of the F1 cross between homozygous green (GG) and homozygous yellow (gg) pea plants. All offspring (100%) have the genotype Gg and the phenotype is green.

Step 5: Take two heterozygous offspring from the F₁ generation and cross them

Step 6: Combine the gametes in each box of the Punnett grid

Punnett grid showing the results of the F2 generation

Phenotype ratio is 3:1 green:yellow, Genotype ratio is 1 GG: 2 Gg: 1 gg

• Plants can sexually reproduce in different ways:
  • Some plants have the male and female reproductive parts within the same flower
  • Others have male flowers and female flowers on the same plant
  • Others have different male and female plants
• Plants with male and female reproductive parts on the same plant can be capable of self-pollination and self-fertilisation
• Farmers and ornamental plant growers can control the way their plants reproduce by artificially pollinating them
  • If a grower thinks a trait is useful or profitable they may choose to self-pollinate the favoured plants to keep the desirable traits in the next generation
  • Growers can also cross-pollinate by artificial pollination between different plants with favoured traits, with the goal to create new generation of plants will possess the desirable traits from both parent plants
  • Genetic crosses can be used to predict and plan for these outcomes