Chapter 12 Chromosomal Patterns of Inheritance

Autosomes Sex Chromosomes Our Genome’s Basic Organization. Chromosomes 1-22Chromosomes X, Y

P: AABB x aabb Gametes: AB, ab F1: AaBb Gametes: AB, Ab, aB, ab F2: AABB, AABb, AAbb, AaBB, AaBb, Aabb, aaBB, aaBb, aabb What does this correlate to in the real world? A single letter corresponds to a particular trait of interest. To distinguish between different ‘flavors’ of the trait, we use differing letters (capital vs. lowercase). –Height: T and t, because there are only two possible flavors of height in our previous examples: Tall and Short. –What if there was more than one allele? We couldn’t use simply capital and lowercase letters. –Perhaps T1, T2, T3? –The nomenclature is not important – what is important is the fact that we are representing real-world phenomenon with arbitrary ‘placeholders’. Recalling Mendel

P: AABB x aabb Gametes: AB, ab F1: AaBb Gametes: AB, Ab, aB, ab F2: AABB, AABb, AAbb, AaBB, AaBb, Aabb, aaBB, aaBb, aabb Recalling Mendel

AABB AA BB A is the name we give to a trait of interest. How does a trait get determined? By a gene. So A also represents a gene. A gene is just a sequence of DNA along a chromosome. So A also represents a sequence of DNA. When our sequences differ in genes (which they do), we call each version of the sequence an allele. So A represents an allele, too. We distinguish between different alleles by changing something about the representing letter. If there are only 2 alleles for the gene (like mendel’s traits), then we can simply use capital and lowercase letters.

Non-Mendelian Genetics But remember that A (or a, or Y, or X R or R 1 ) all of these ultimately represent a sequence of DNA! Sometimes, we’re interested in a sequence of DNA that is longer than just 1 gene. Sequences of DNA that encompass multiple genes are called genetic loci, or just loci. And keep in mind that a chromosome is simply one huge long stretch of single-stranded DNA. So any level of DNA can potentially be of interest to a genetics situation, including chromosomes. Aa (a gene) R S (a locus) (a chromosome)

Sex Chromosomes and Sex-Linked Traits When we’re concerned with the movement of entire chromosomes from parents to offspring, we can represent them the same way we represent genes - with a single letter. X chromosome Y chromosome XY = male XX = female We care about genes on the X chromosome, because males only have a single copy of them (a single factor / allele), while females have 2. These leads to interesting genetics phenomena. XX XY

Sex Chromosomes and Sex-Linked Traits We call traits associated with genes on the X and Y chromosomes “Sex-Linked”. Because the X chromosome is bigger (and therefore has many more genes than the Y chromosome), many of the sex-linked genes we learn about are located on the X chromosome. We therefore call these “X-Linked” genes (which of course have associated “X-linked” alleles) Here, notice that males have only a single allele for the gene pointed out in red. For mom, it’s just like any other gene; she can be dominant or recessive. But Dad is different – he has only a single allele for the gene. XX XY

Sex Chromosomes and Sex-Linked Traits That means if dad has a recessive allele, he will express its phenotype; there is no compensating dominant allele. So dad can never be a carrier. He either has the X-linked trait, or he doesn’t. As an example, we’ll use fruit flies. R = red eyes r = white eyes Because we know the gene for eye color is on the X chromosome, we use the following notation: X R, X r Let’s do a few of these crosses on the marker board. YX R X R X R