#137 Summary of inheritance

Chromosomes are long thread of DNA made up of strings of genes. In a diploid cell, each of a pair of homologous chromosomes carries the same genes in the same position. A diploid cell therefore has 2 copies of each gene.

Things to remember: 

• Gametes have only one set of chromosomes , and so they have only one copy of each gene.

• Different forms of a particular gene are called alleles. They may be dominant of recessive. The genotype of an organism tells us the alleles of genes that it carries. If the 2 alleles of a gene are the same in the organism, then it is homozygous. If they are different, it is heterozygous.

• If 2 heterozygous organisms breed together, we expect a 3:1 ratio of offspring showing the dominant characteristic to offspring showing the recessive characteristic. If one parent is heterozygous and the other is homozygous recessive, we expect to see a 1:1 ratio in the offspring.

• Variations is caused by genes and environment. Continuous variation, such as human height, has no distinct categories and is usually caused by both genes and environment. Discontinuous variation, such as human blood groups, involves a small number of discrete categories and is caused by genes alone.

• New alleles of genes, or changes in categories chromosomes, can be caused by mutation. Most mutations are harmful. Ionising radiation and certain chemicals increase the risk of mutation happening.

• In a population of organisms, those with the characteristics best adapted to the environment are most likely to survive and reproduce. This is called natural selection.

• It the environment changes, or if a new advantageous allele appears, natural selection can leas to change over many generations. This is called evolution.

• Sickle cells anaemia is caused by recessive allele of the gene for haemoglobin. People who are homozygous recessive often die before they can reproduce. People who are homozygous dominant have a greater chance of getting malaria if they live in places where this disease is present. People who are heterozygous have a selective advantage, because they are less likely to get malaria. Natural selection therefore maintains this allele in the population in parts of the world where people may be killed by malaria.

• Human select plants and animals with desirable characteristics and breed from them. Over many generations, this produces new strain of plants or animals with features that’s we require. This is called artificial selection.

• Genetic engineering involves taking a gene from one species and inserting it into another. This has been done with the human insulin gene, to give bacteria that produce insulin for harvest an sale, for use by people with diabetes. 

Key definitions 

Chromosome	A thread of DNA, made up of genes.
Allele	An alternative form of a gene. Pairs of alleles occupy the same relative positions on chromosome pairs.
Gene	A section of DNA, which codes for the formation of a protein controlling a specific characteristic of the organism.
Haploid nucleus	A nucleus containing a single set of unpaired chromosomes, e.g. in sperm and ova (eggs). In humans, the haploid number is 23.
Diploid nucleus	A nucleus containing pairs of chromosomes, e.g. in somatic (body) cells, In humans the diploid number is 46.
Genotype	The genetic make-up of an organism, e.g. Tt, where T and t are alleles of a gene.
Phenotype	The characteristics visible in an organism, controlled by the genotype, e.g. a tall plant or a dwarf plant.
Homozygous	Having a pair of identical alleles controlling the same characteristics, e.g. TT, where T=tall. The organism will be pure-breeding for that characteristics.
Heterozygous	Having a pair of dissimilar alleles for a characteristic, e.g. Tt.
Dominant	A gene, e.g. T, that always shows in the phenotype of an organism whether the organism is heterozygous (Tt) or homozygous (TT).
Recessive	A gene, e.g. t, that only has an effect on the phenotype when the organism is homozygous (tt)