Inheritance

Define the terms: gene, allele, loci, genotype, phenotype (trait), dominant, recessive, autosome, sex chromosome, homozygous, heterozygous, monogenic, wild-type, mutant, haploid, diploid, zygote.

• Gene- A section of DNA that encodes for protein or RNA. It can be passed down through the generations.
• Allele- Variants of a gene that can be found in the population.
• Loci- The area of the DNA where a particular gene can be found.
• Genotype- The set of alleles that a person has.
• Phenotype- The appearance or traits that a person has.
• Dominant- An allele that is expressed either alone or in a pair.
• Recessive- An allele that is only expressed if there are no dominant alleles.
• Autosome- One of the 22 non-sex chromosomes.
• Sex chromosome- The X or Y chromosome.
• Homozygous- Both alleles for that trait are the same.
• Heterozygous- The alleles for that trait are different.
• Monogenic- Only one gene codes for that trait.
• Wild-type- A trait or gene that is of the typical form found in nature.
• Mutant- A form of a trait or gene that is not typically found in nature.
• Haploid- A cell with half of the normal number of chromosomes (23 in humans)
• Diploid- A cell with the normal number of chromosomes (46 in humans)
• Zygote- The cell that forms when the egg and sperm meet for the first time.

Have a basic understanding of Mendelian inheritance and segregation.

Mendelian inheritance stipulates that each individual has two copies of each gene: one from each parent. During the formation of germ cells, the copies of each gene are segregated out, so each germ cell will only have one copy of a given gene. The copy that is inherited by a particular germ cell is random. When the sperm and egg meet, the randomly-assorted gene copies in the sperm and the randomly-assorted gene copies in the egg come together to form the genetic makeup of the zygote. The genotype of the zygote thus depends on which genes it inherits from the sperm and which genes it inherits from the egg, and the phenotype of the zygote depends on which genes get expressed.

Often, when talking about Mendelian inheritance, we are normally only looking at a specific gene for a specific disorder or trait that we are interested in. For example, cystic fibrosis is an autosomal recessive disorder, meaning that it is inherited on an autosome (non-sex chromosome) and will only be expressed if both copies of the gene have the recessive allele. If you know the genotype of the parents, you can guess the likely genotypes and phenotypes of the children. Let's say that the dominant allele on the CFTR gene (the gene involved in cystic fibrosis) is C, and the recessive allele is called c. Hence, if you have two parents that are both Cc, these are the combinations that could result:

	C	c
C	CC	Cc
c	Cc	cc

Note that in this particular example, there is a 1/4 chance that the genotype of the child is CC, 2/4 = 1/2 chance that the genotype is Cc, and a 1/4 chance that the genotype is cc. Since cystic fibrosis is a recessive trait, the child will only have the cystic fibrosis phenotype if they have two recessive alleles (i.e. a genotype of cc).

Also note that some genetic traits are sex-linked, meaning that they are passed down on one of the sex chromosomes (usually the X chromosome). If this is the case, you will also need to keep track of a) which chromosome is getting passed down and b) if it is the chromosome of interest, whether it is carrying the dominant or recessive allele for that trait. You can do this by using subscripts or superscripts (e.g. X^aY).

Recognise pedigree symbols & be able to determine modes of inheritance in pedigree charts.
Understand the possible genotypes within pedigree charts; label & explain genotypes & phenotypes.

A pedigree chart is a useful way of tracking how characteristics are inherited within families. Pedigree charts basically just look like family trees. The shape of the "leaves" on the tree represent sex- usually squares are males, circles are females, and diamonds are unspecified. The colour of the "leaves" represent whether or not the person in question has been affected by that trait- black means that they have the trait, whereas white means that they do not have that trait. Some pedigrees might even shade half the box if the person is a carrier (has a recessive allele that is not being expressed), but not all. Therefore, it is usually really easy to tell what phenotype a person of interest has (by the colour of their "leaf"), but it is not always easy to tell the genotype as someone could be either heterozygous or homozygous for a dominant allele.

Have an understanding of how human disorders are associated with single genes.

I've already mentioned cystic fibrosis as an example of a disorder that is associated with a single gene, but it is not the only monogenic disorder out there. Huntington's Disease is autosomal dominant, haemophilia is X-linked recessive, and vitamin D-resistant rickets (i.e. rickets that doesn't get fixed by simply supplementing vitamin D) is X-linked dominant. This is by no means an exhaustive list- this is just to give you an idea.