Rapid Review

Genetics

Below are some basic genetic principles.

• Inherited characteristics (traits) are passed from generation to generation by means of genes.
  
• Genes are comprised of DNA and are contained in chromosomes.
  
• A locus is a particular place along the length of a certain chromosome where a given gene is located.
  
• Alternative versions of genes that occupy a certain locus are called alleles.

Mendelian Inheritance

For a given trait, an organism inherits two genes (or alleles), one from each parent. The genes are located at the same locus on a homologous pair of chromosomes. The two alleles may be the same, in which case the organism is said to be homozygous for that trait, or different, in which case the organism is said to be heterozygous for that trait. If the organism is heterozygous, then usually one of the alleles is dominant and is fully expressed in the phenotype, and the other allele is recessive and is not expressed.

Law of Segregation

During gametogenesis, the two alleles for each trait separate and go into different gametes so that each gamete contains only one allele.

If the parent is homozygous, each gamete will carry the same allele. If the parent is heterozygous, 50% of the gametes will carry one allele and 50% will carry the other allele.

Example:
In peas, there are two alleles for seed color. One allele (Y) produces yellow seeds. The other allele (y) produces green seeds. A heterozygous pea plant has the genotype Yy and produces gametes of which 50% will carry the Y gene and 50% will carry the y gene.

The law of segregation is concerned with one trait, one locus. Mendel also derived a rule for the inheritance patterns observed when there are two traits of interest.

Law of Independent Assortment

Each pair of alleles segregates independently of other pairs of alleles provided the different pairs of alleles are on different pairs of homologous chromosomes.

Example:
Pea seeds exhibit the traits of color and texture. They can be yellow or green (the trait of seed color), and they can be smooth (R allele) or wrinkled (r allele) (the trait of texture). The loci for these two traits are on different pairs of homologous chromosomes. A pea plant heterozygous for both of these traits has the genotype YyRr. During meiosis (gametogenesis), the genotypes of the gametes are distributed as follows:

Genotype	YR	Yr	yR	yr
Frequency	25%	25%	25%	25%

Monohybrid Cross

A monohybrid cross is a breeding experiment that is concerned with differences in only one trait. If two heterozygotes for seed color are crossed, the expected result is:

		Gametes from one parent
		Y	y
Gametes from other parent	Y	YY	Yy
	y	Yy	yy

Y is dominant and y is recessive. Thus, the ratio of yellow to green seeds is 3:1, a result of the law of segregation.

Dihybrid Cross A dihybrid cross is a breeding experiment that is concerned with differences in two traits. If two heterozygotes for seed color and seed texture are crossed, the expected result is (note that R is dominant and r is recessive):

		Gametes from one parent
		YR	Yr	yR	yr
Gametes from other parent	YR	YYRR yellow, smooth	YYRr yellow, smooth	YyRR yellow, smooth	YyRr yellow, smooth
	Yr	YYRr yellow, smooth	YYrr yellow, wrinkled	YyRr yellow, smooth	Yyrr yellow, wrinkled
	yR	YyRR yellow, smooth	YyRr yellow, smooth	yyRR green, smooth	yyRr green, smooth
	yr	YyRr yellow, smooth	Yyrr yellow, wrinkled	yyRr green, smooth	yryr green, wrinkled

The ratio of the different phenotypes is 9:3:3:1 as displayed below.

Phenotype	Yellow, smooth	Yellow, wrinkled	Green, smooth	Green, wrinkled
Frequency	9/16	3/16	3/16	1/16

Sex Determination

In humans and many other animals, sex is determined by the sex chromosomes, X and Y. Males have an X and a Y chromosome, while females have two X chromosomes. As shown below, the probability of having a son or daughter is always about 50%.

		Father's gametes
		X	Y
Mother's gametes	X	XX female	XY male
	X	XX female	XY male

Note that in males, the X chromosome always comes from the mother, while the Y chromosome always comes from the father.

Sex-linked Inheritance

Genes that are on the sex chromosomes are said to be sex-linked. Genetic disorders that are sex-linked are often characterized by mothers being carriers and their sons being affected with the disorder.

The X chromosome is much larger than the Y chromosome and so carries more genetic information. For instance the genes for hemophilia and color blindness, both of which are recessive, are located on the X chromosome. The Y chromosome does not have loci for these genes or their alleles; therefore, when a male inherits an X chromosome from his mother containing the hemophilia gene, the gene is expressed resulting in the male being hemophilic.

Mutations

A mutation is a change in the sequence of nucleotides in a gene. Mutations can occur spontaneously as a result of errors during DNA replication, repair, or recombination. Ionizing radiation and certain chemicals can also cause mutations.

Type of mutation	Description
Base-pair substitutions	Replacement of a base-pair by another base-pair. Yet there may be no effect on the protein coded for since the genetic code is redundant. If a different amino acid is incorporated into the protein, the effect may still be negligible if the new amino acid has similar properties to the one it has replaced, or if the location of it is far from the active site, etc. of the protein. However, some substitutions are significant, e.g. sickle-cell anemia is due to a single base-pair substitution mutation that occurred many generations ago.
Base-pair insertions and deletions	Additions or losses of one or more base-pairs. These are usually more deleterious than substitutions. This is because mRNA is read as triplets of bases. If three nucleotide pairs are inserted or deleted, the problem may not be very significant. However, if one or two base-pairs are inserted or deleted, a frame-shift occurs causing all bases downstream to be read incorrectly. A non-functional protein is likely unless the frame-shift mutation occurs near the end of the gene.