What are the chances? • Do people with brown hair always have brown eyes? • Do people with brown hair have a higher chance of having brown eyes? • Is a round pea seed always yellow? Can a short plant have purple flowers?
INDEPENDENT ASSORTMENT • The inheritance of one gene does not influence the inheritance of another. • Mendel named this: The Law of Independent Assortment • In meiosis, the chromosomes line up randomly on the equator to be separated. • If your parents are heterozygous for any traits, this leads to lots of possibilities!
How did Mendel figure this out? • Through experiments… • He crossed two different plants • Each was true-breeding for 2 different traits • P gen: rryy (wrinkled & green) x RRYY (ROUND & YELLOW) • F1 gen: RrYy • F2: a bunchofpossibilities!
Aligning the Punnett Square • F1 gen now has: RrYy • Distribute the 1st letter of the 1st set to each of the 2 letters in the 2nd set. • Distribute the 2nd letter of the 1st set to each of the 2 letters in the 2nd set R r Y y • 4 possibilities: RY, Ry, rY, ry
Patterns A Het x Het dihybrid cross yields the typical phenotypic ratio of: 9: 3: 3: 1
Now You Try! In humans, the gene that causes a unibrow (u) is recessive to not connected eyebrows (U); the gene for thick lips (T) is dominant over the gene for thin lips (t). If a male that is homozygous for not connected eyebrows and heterozygous for thick lips mates with a woman who has a unibrow and is heterozygous for thick lips, what is the phenotypic ratio of the offspring?
How do you set it up? U = not connected eyebrows u = unibrow T = thick lips T = thin lips 1. Make one column for each possible different combination of alleles from dad dad 2. Make one row for each possible different combination of alleles from mom mom 3. Fill in the boxes • Genotypes --Dad = UUTt --Mom = uuTt • Possible Different Combinations of Alleles --Dad = UT or Ut --Mom = uT or ut
Results of the Cross UuTT UuTt Only include the phenotypes present in the results. UuTt Uutt Phenotypic Ratio 3 not connected eyebrows & thick lips : 1 not connected eyebrows & thin lips
Summing It Up: Mendel’s Principles 1. Parents pass on characteristics, sexually, through genes to their offspring 2. When there are multiple alleles (appearances) for one gene, some are dominant & some are recessive • During formation of parental gametes, alleles are segregated into separate gametes. Each parent is then able to pass ONE allele to the child. The child therefore gets ONE allele from EACH parent • The chromosomes (and therefore alleles) from each parent arrange themselves independently during meiosis
11-3 Exploring Mendelian Genetics Going Beyond Simple Dominance: Incomplete Dominance Codominance Multiple Alleles Polygenic Traits X- linked (or sex linked) Traits
Incomplete Dominance In some cases, neither allele truly dominates over the other. No allele is really dominant or recessive The heterozygous genotype shows a MIX of the two traits. Example- Four O’Clocks R- gene for red flowers, W- gene for white flowers: RR- red, WW- white, RW- pink
Example: Snapdragons RR = Red WW = White All RW = Pink
Codominance In some cases, both alleles are dominant. No allele is really recessive. The heterozygous genotype shows BOTH of the two traits. Example- Chicken feathers B- gene for black feathers, W- gene for white feathers: BB- black, WW- white, BW- “erminette” Black and White!
Multiple Alleles Many genes have more than just two alleles for a trait Remember, you can still only have 2 alleles at a time. It is still just ONE gene, but lots of possibilities Example: Alleles for rabbit fur C-full color, dominates over cch, ch, c cch – chinchilla, dominates over ch and c ch- himalayan, dominates over c c- albino, recessive to all
Practice Cross cchch x cc What are the phenotypes of the parents? light gray x albino Fill in the Punnett Square. What is the probability that an offspring will be albino (albino = cc)? 0 out of 4 = 0% What is the probability that an offspring will be himalayan (himalayan = chch, chc)? 2 out of 4 = 50% cch ch c Light gray Himalayan chc cchc Light gray Himalayan c cchc chc
Polygenic Traits Many traits result from the interaction of several genes. Multiple genes, perhaps on different chromosomes even, produce one phenotype Polygenic traits can produce a large range of phenotypes Examples: human skin color (at least 4 genes), human eye color, human height
Capital letters = dark; small letters = light; more dark alleles = darker!!
Height in Humans Range of phenotypes resulting from polygenic trait
Human Blood Types Exhibit 3 special situations Multiple alleles - A, B, O Polygenic - one gene controls type, another gene controls rH factor (+, -) Codominance - A and B are codominant but both dominate over O
Linkage • Really it is the chromosomes that are segregated independently, not necessarily individual genes. • Some genes are LINKED if they are on the same chromosome • Ex: you get all of the genes on chromosome 1 from your mom if you get her chr.1 • Is that always the case though? What do you know might happen?
Crossing Over • Depending on how FAR APART genes are on chromosomes, they may be switched during meiosis • Occurs during Prophase I • Must be homologous chromosomes
Gene Maps Where actual genes are located on chromosomes. Discovered by a student working in molecular lab at Columbia in 1931. By looking at statistical inheritance patterns, you can calculate the “recombination frequency” of alleles. If genes are far apart, crossing over is more likely
Sex-linked Traits Often called X- linked traits Trait can be dominant or recessive Probability of inheritance is altered because the trait is on the X chromosome Females- XX Males- XY
Examples of X-linked traits Colorblindness Hemophilia Duchenne Muscular Dystrophy Female: Carries hemophilia gene Male: Has hemophilia Red = hemophiliac gene