Genetics: Inheritance I (Mendelian)

I. Some background and definitions A.Chromosome B.Locus- a specific location on a chromosome (pl. “loci”) C.Gene- each gene is found at a specific locus on a specific chromosome D.Allele

The gene that codes for the beta-subunit polypeptide of a G-protein is located on this chromosome (15) at 15q21.2 The gene that codes for a tyrosine kinase receptor (type 3) used by neurons is located at 15q25.

I. Some background and definitions A.Chromosome B.Locus C.Gene D.Allele E.Genotype- the alleles an organism has at a gene

I. Some background and definitions A.Chromosome B.Gene C.Allele D.Genotype- the alleles an organism has at a gene E.Phenotype- the anatomy and physiology of an organism; the expression of genes in the organism; the way the organism “looks” and “works.”

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype: a.Each of us inherits 23 chromosomes from mom and 23 from dad; they are homologous.

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype: a.Each of us inherits 23 chromosomes from mom and 23 from dad; they are homologous. *remember that we are using humans as the example. Humans are diploid, and we have 23 pairs of homologous chromosomes. Each species has a characteristic number of chromosomes. For example, dogs have 39 pairs, mosquitoes have 3 pairs, and cabbage plants have 9 pairs. Haploid organisms do not have homologous pairs; they have only one copy of each chromosome (therefore one copy of each gene).

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype: a.Each of us inherits 23 homologous chromosomes b.Genotype: Each pair of homologues has the same genes on the same location, but may have different ALLELES for those genes. For example, at the (brown-blue)eye color gene, mom could have given you a blue (b) allele and dad could have given you a Brown (B) allele. If this were the case, your GENOTYPE would be Bb. (What would your genotype be if you got b from mom and b from dad?)

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype: a.Each of us inherits 23 chromosomes from mom and 23 from dad; they are homologous. b.Genotype c.Phenotype: If you get two of the same allele (ex, bb), you must express (show) that trait- you will have blue eyes. However, often when you get two different alleles, one expresses and the other does not. For example, if your genotype is Bb, only the Brown allele expresses, and you will have brown eyes. This is your PHENOTYPE.

Brown eyesBlue eyes These are phenotypes; what are their genotypes?

Brown eyesBlue eyes BB or Bbbb

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! a.Heterozygous and homozygous- when someone has 2 of the same allele at a gene, we say they are HOMOZYGOUS for that gene. When someone has 2 different alleles at a gene, we say they are HETEROZYGOUS for that gene. An individual will be homozygous for some genes, and heterozygous for other genes. For example, you could have brown eyes (Bb), and not be able to roll your tongue (tt).

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! a.Heterozygous and homozygous- when someone has 2 of the same allele at a gene, we say they are HOMOZYGOUS for that gene. When someone has 2 different alleles at a gene, we say they are HETEROZYGOUS for that gene. b.Dominant and recessive alleles- When someone is heterozygous for a gene, and only one allele expresses, we say it is the DOMINANT allele. The one that does not express is RECESSIVE.

This example considers the genetics of pea color.

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! a.Heterozygous and homozygous b.Dominant and recessive alleles c.Incomplete dominance and codominance: Sometimes there is no dominance by one allele in heterozygotes. Instead, both get to express. i.Incomplete dominance- alleles blend their influence

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! a.Heterozygous and homozygous b.Dominant and recessive alleles c.Incomplete dominance and codominance: Sometimes there is no dominance by one allele in heterozygotes. Instead, both get to express. i.Incomplete dominance- alleles blend their influence ii.Codominance- both alleles express fully

Blood types

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! a.Heterozygous and homozygous b.Dominant and recessive alleles c.Incomplete dominance and codominance d.Polygenic traits- phenotypes determined by the interaction of more than one gene. Many traits are polygenic! For example, see book for skin color determination and the next slide for eye color!

Example of polygenic trait: eye color Eye color is determined by the interaction of at least 2 genes: Brown-blue and Yellow-absent. Yellow coloration (Y) is dominant to absent or no coloration (y). If genotype at Blue-brown is And genotype at Yellow-absent is Then phenotype will be Bb or BBYy or YYHazel Bb or BByyBrown bbYy or YYGreen bbyyBlue

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! a.Heterozygous and homozygous b.Dominant and recessive alleles c.Incomplete dominance and codominance d.Polygenic traits e.Epistasis- when one gene has a permissive effect on another

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! a.Heterozygous and homozygous b.Dominant and recessive alleles c.Incomplete dominance and codominance d.Polygenic traits e.Epistasis- when one gene has a permissive effect on another f.Pleiotropy- when one gene affects many aspects of phenotype

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! 3.Predicting genotypes and phenotypes: punnet squares (laboratory activity). a.Monohybrid crosses only consider one gene at a time

If mom is Aa for a particular gene, each of her eggs will have EITHER the A or a allele. Same for dad’s sperm if he’s Aa What allele will each egg carry if mom is AA? aa?

P generation: the parents F1 generation: the offspring (in this example, all F1 are Pp, and we are mating two “siblings” to produce the: F2 generation: the grandchildren (offspring of the F1)

Here’s an example using alleles that show incomplete dominance

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! 3.Predicting genotypes and phenotypes: punnet squares. a.Monohybrid crosses only consider one gene at a time b.Test crosses allow you to determine the genotype of an individual with the dominant phenotype of a gene. So, in this case you are not trying to predict the outcome of an F1 generation. Instead, you are using the F1 generation to figure out the genotype of a parent with a dominant phenotype. To do so, mate the “unknown” parent with another parent of known genotype: a homozygous recessive.

Example: test cross In laborador retrievers, brown (chocolate) color (b) is recessive to black color (B) You own a male black lab, Rocky (short for RockEater). What are Rocky’s possible genotypes? Let’s say you mate Rocky with your neighbor’s chocolate lab, Lucinda. What is Lucinda’s genotype? If Lucinda has 20 puppies, and 10 are chocolate, do you know what Rocky’s genotype is? What if all 20 are black? What if only 2 are chocolate? So, in a test cross, you are trying to figure out the genotype of a dominant phenotype parent by mating that parent with a recessive phenotype parent. The outcome of the offspring leads you to your answer.

“Rocky” “Lucinda”

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual 1.Genotype and Phenotype 2.Some more definitions, hooray! 3.Predicting genotypes and phenotypes: punnet squares. a.Monohybrid crosses only consider one gene at a time b.Test crosses allow you to determine the genotype of an individual with the dominant phenotype of a gene. c.Dihybrid crosses allow you to consider two genes at a time

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual B.In a population- there a typically many alleles for each gene. Each individual can only get 2.

In the human population, there are 3 major alleles for the ABO Blood surface protein gene: A I B I i codominant Recessive to both

If your genotype isThen your phenotype (blood type) will be A A A I I or I i A B B B I I or I i B A B I I AB iiO

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual B.In a population- there a typically many alleles for each gene. Each individual can only get 2. C.Mendel -described two principles which can now be explained by the mechanics of meiosis: -Principle of segregation- alleles are separated from each other

II. The basics of how genetic inheritance works (using humans as the example) A.In the individual B.In a population- there a typically many alleles for each gene. Each individual can only get 2. C.Mendel -described two principles which can now be explained by the mechanics of meiosis: -Principle of segregation -Independent assortment- genes that are on separate chromosomes separate from each other

III. Disorders caused by “faulty” alleles A.Deleterious dominant alleles- will only survive in a population if they allow their holders to survive to reproductive age

III. Disorders caused by “faulty” alleles A.Deleterious dominant alleles- will only survive in a population if they allow their holders to survive to reproductive age Ex, Huntington’s disease, doesn’t strike until 40’s.

III. Disorders caused by “faulty” alleles A.Deleterious dominant alleles Ex, Huntington’s disease, doesn’t strike until 40’s. B.Deleterious recessive alleles- survive in a population better than dominant alleles because they can “hide” in heterozygotes. Heterozygotes are called “carriers.”

III. Disorders caused by “faulty” alleles A.Deleterious dominant alleles Ex, Huntington’s disease, doesn’t strike until 40’s. B.Deleterious recessive alleles- survive in a population better than dominant alleles because they can “hide” in heterozygotes. Heterozygotes are called “carriers.” Ex, cystic fibrosis

III. Disorders caused by “faulty” alleles A.Deleterious dominant alleles Ex, Huntington’s disease, doesn’t strike until 40’s. B.Deleterious recessive alleles- survive in a population better than dominant alleles because they can “hide” in heterozygotes. Heterozygotes are called “carriers.” Ex, cystic fibrosis *When deleterious recessive alleles are X-linked, the disease condition is more common in males Ex, hemophilia *Inbreeding tends to “bring out” recessive traits; why would that be?

IV. Tracking alleles through generations: pedigrees Laboratory activity