BSC 1005 Chapter 6 notes

Chapter 6 : Genetics
Genetics is the study of inheritance. First studied by Gregor Mendel in 1850-1870 using peas. Essay 6.1.

6.1 Inheritance of Traits

A gene is a section of DNA that carries instructions for making proteins. Genes are on chromosomes and you inherit one set of chromosomes from each parent. In humans, one set is 23 chromosomes. When the sperm (with 23 chromosomes) fertilizes the egg (with 23 chromosomes) they form a zygote or fertilized egg that has 46 chromosomes. So you end up with homologous pairs of chromosomes. Fig 6.3

A mutation is a change in the DNA. Mutations form different versions of a gene called alleles ( for example there are alleles for blond hair, brown hair, red hair, black hair). When the egg and sperm are formed by meiosis the alleles separate at random (independent assortment). This produces over 8 million different types of sperm and egg.
The sperm that fertilizes the egg is also picked at random, and this random fertilization means that in humans one pair of parents could theoretically produce almost 70 trillion different offspring. The main exception to this is identical twins, which come from the same fertilized egg, so are genetically identical.

6.2 Mendelian Genetics

Genotype is the genetic makeup of an individual eg Ff or FF. Fig 6.10.

Phenotype is the physical appearance eg carrier of cystic fibrosis.
Monohybrid cross - only one trait is studied. Typically one allele is dominant. That means that it shows up in the phenotype whether you have one copy of it, or two copies. A Recessive allele only has an effect if you inherit two copies eg ff (cystic fibrosis Fig 6.10). If two carriers have children, each child has a 25% of being born with the disease. Because the disease is recessive, it can skip several generations.

Genotype		Phenotype
F F	Homozygous dominant	No cystic fibrosis (Normal)
F f	Heterozygous	Carrier (has no symptoms but carries the recessive allele)
f f	Homozygous recessive	Cystic fibrosis (has symptoms)

Cystic fibrosis affects the lungs. To predict how many offspring will have a particular genotype you use a Punnett square

Some genetic diseases are caused by a dominant gene. The only way someone can have the disease is if one of their parents has it (and one of their grandparents etc). It cannot skip generations. Heterozygous individuals suffer from the disease. If a heterozygous person has children with a normal (homozygous recessive) person, each child has a 50% chance of inheriting the disease. For example Huntington's disease Fig 6.11 a) which affects the nerves.

Dihybrid cross

Each trait is inherited independently. There are four possible phenotypes in the offspring. Fig 6.12. If two heterozygous individuals are crossed ( eg RrYy : Round, yellow peas ) the result is a ratio of :

        9      :       3       :       3       :       1
     Yellow,         Green,         Yellow,          Green,    
     round seeds     round          wrinkled         wrinkled

6.3 Quantitative Genetics

Traits like height, weight, eye color and intelligence are quantitative traits, which means they have continuous variation. Typically in a population the graph shows a normal distribution or bell curve. Often quantitative traits are affected by several genes: they are polygenic. So there is a range of variation in the population. The offspring of heterozygous parents can be very different from either parent, so average sized parents can have a very tall child. Quantitative traits are also often affected by the environment eg skin color and intelligence.

Heritability is a measure of how much a trait is affacted by genes, rather than the environment. For example in cows the heritability of milk production is 30%, so 30% of the difference in milk production from one cow to another is caused by genes, the other 70% is due to environmental differences. Estimates of the heritability of intelligence in humans range from 42% to 72%, with an average estimate of about 60%. These estimates are obtained by correlating parents and children's IQ, and also by studying identical twins and nonidentical twins.

6.4 Genes, Environment and the Individual

In mice body weight has a heritability of 90%, however body weight is also obviously affected by the environment. Fig 6.18.

Experiments with mice that are tested in mazes (Table 6.2) show that although mice may differ genetically in their ability to navigate a maze, the environment in which they are raised is even more important than their genes. It is likely that the same is true of human intelligence.

Last edited October 2009, by David Byres, dbyres@fscj.edu