Presentation on theme: "WHAT IS GENETICS? GENETICS is the study of how traits are passed from parent to offspring in the form of Genes."— Presentation transcript:
WHAT IS GENETICS? GENETICS is the study of how traits are passed from parent to offspring in the form of Genes.
HISTORY! Gregor Mendel Born 1822 Austrian Monk Examined reproduction of pea plants
Plants reproductive organs are called FLOWERS A flower has both male and female parts. The pea plants Mendel was working with were typically TRUE-BREEDING, meaning they self-pollinated EX, TALL pea plants would always be pollinated by tall pea plants and produce tall offspring!
WHAT WE KNOW (MENDEL DIDN’T) Genes – control a heritable feature (characteristic); Example: Hair color, seed shape, height; Allele – controls the variation of a feature (trait). Example: brown, blonde, black hair
REVIEW TIME: What are homologous chromosomes???
Homologous chromosomes may…… -Both have the same alleles HOMOZYGOUS (aka: pure or true-breeding) -Both have different alleles HETEROZYGOUS (aka: hybrid)
Mendel’s Idea Cross two pea plants with different contrasting traits! Ex: First cross : Crossed true breeding purple with true-breeding white plants. Called offspring F1 Generation Results were that offspring were_100% PURPLE_ Had the white allele disappeared????
Mendel’s Law of Dominance some alleles over power others. So even if both alleles are present, we only “see” the dominant one. the “hidden” allele is called recessive This only applies to SOME genes, not all
Second cross two of the purple F1 Offspring Called offspring the F2 Generation Results - 75 % purple - 25 % were white White trait had reappeared!
“The Traits (genes) Mendel looked at
Mendel’s Law of Segregation during meiosis, the pair of alleles in a parent will separate. Only ONE allele for EACH TRAIT will pass from each parent to the offspring
Ex. sugar beet preference. dominant allele (A) prefers sugar beets recessive allele (a) does not. Heterozygote produces gametes 50% chance Get A Get a Question: If a heterozygous sugar beet eater marries a non-sugar beet eater, what possible offspring could they have?
Mendel’s Law of Independent Assortment Alleles for different genes are passed to offspring independently of each other. The result is that new combinations of genes present in neither parent is possible. How many allele combinations could the following genotype produce? RRYY RRYy RrYy
Genetic Terms Diploid (2n)- Two sets of chromosomes. Somatic Cells Haploid (n)- One set of homologous Chromosomes (gametes) Egg- Female haploid gamete Sperm- male haploid gamete
Parent – Seriously, you should know this Meiosis – Cell division that produces haploid gametes Testes – Site of male meiosis Gamete – Haploid sex cell (sperm, egg, pollen)
Zygote- Single cell (result of sperm and egg) Progeny - Offspring Offspring – see above Fertilization – gametes fuse into zygote Ovary- site of female meiosis - eggs
Genotype: the alleles that an organism has. - alleles are abbreviated using the first letter of the dominant trait. - capital letter represents the dominant - ex: P for purple flower allele - lower case represents the recessive. - ex: p for white flower allele
All diploid organisms have two alleles for each trait: Can be two of the same alleles Ex: PP or pp called Pure or Homozygous. OR Can be two different alleles Ex: Pp described as Hybrid or Heterozygous
Phenotype: physical appearance Examples: brown hair, widows peak, purple flowers the trait that “wins” in the case of complete dominance; depends on the combination of alleles GENOTYPE
MENDEL’S CROSSES P Generation: “parents;” F1 Generation offspring of P generation F2 Generation offspring of F1 generation Punnet Squares How we show allele combinations in crosses
Allele in Egg 1 Zygote formed if sperm 1 fertilizes egg 1 Allele in Egg 2 Allele in sperm 1 Allele in sperm 2 Zygote formed if sperm 2 fertilizes egg 1 Zygote formed if sperm 1 fertilizes egg 2 Zygote formed if sperm 2 fertilizes egg 2
Monohybrid Cross Tall vs. Short Example Tall allele TShort allele t P CrossTTxtt F1 Generation Genotypes Phenotypes TT t t
F2 Generation F1Generations 100% Tt TtxTt F2 Generation Genotypes- Ratio = Phenotypes- Ratio = t t T T
Sample Problems Homozygous Tall x Heterozygous Tall Heterozygous Tall x Homozygous Short
Probability Probability is only the LIKELIHOOD of an event happening. It does not mean it is what HAS to happen Ex. Coin Toss. Two tosses, always one heads and one tails? What happens when we look at very large samples? Ex. Male/female ratio of a family vs. the world!
INHERITENCE PATTERNS Every gene demonstrates a distinct phenotype when both alleles are combined (the heterozygote) Complete dominance is when both alleles are present, only the dominant trait is seen. Incomplete dominance - when both alleles are present, the two traits blend together and create an intermediate trait
Inheritance Patterns: Co-dominance - when both alleles are present, both traits are visible Different notation: Use first letter of the feature with a superscript for the trait. Example: C W or C R for white petals or red petals;
Women have two X’s but men only have one. How do we deal with the genes on the X chromosome?
Probabilities Question 1: What is the probability of having a female offspring? Question 2: After having 4 sons in a row, what is the probability the next kid will be male? Question 3: What is the probability of having three daughters in a row?
Sex-Linked Traits Refers to traits coded by genes found on the X chromosome Females will have 2 copies of these genes Males will have 1 copy of these genes Significance??? If males get a bad (recessive) allele for a sex- linked trait, THEY WILL EXPRESS THE RECESSIVE TRAIT!
Example – Color Blindness Seeing color (X C ) is dominant to being color blind (X c ) Identify the sex and trait of the following: X C YX C X c X C X C X c X c X c Y
XCXC Cross Number 1: XcXc XCXC Y XCXC XCXC XCXC XcXc XcXc Y XCXC Y What % chance of having color blind daughter? Son?
SEX-LINKED TRAITS COLOR BLINDNESS AFFLICTS 8% MALES AND 0.04% FEMALES.
Test cross: a cross that determines genotype of dominant parent - Cross unknown dominant parent (possibilities BB or BB) with a recessive parent then analyze the offspring. Ex. B- Black Hairb- white hair You are given a black-haired guinea pig and need to determine whether homozygous dominant or heterozygous.
Multiple Alleles Genes may have more than two alleles.
Multiple alleles: Some genes have more than two variations that exist, although we still only inherit 2 Example: Human blood types Three alleles: I A I B i GenotypePhenotype IAIAIAIA A IAiIAiA IBIBIBIB B IBiIBiB IAIBIAIB AB ii0
Polygenic – Multiple genes code for a trait each with 2 alleles Examples in humans: Skin Color Eye Color Height Why so many possibilities??? SKIN PIGMENTATION
Dihybrid cross: A cross that focuses on possibilities of inheriting two traits - two genes, 4 alleles Black fur is dominant to brown fur Short fur is dominant to long fur What is the genotype of a guinea pig that is heterozygous for both black and short fur?
Dihybrid cross: Parent phenotypes: BbSs x BbSs Figure out the possible gametes: Then set up punnett square
Dihybrid cross: BSBsbSbs BS Bs bS bs
Di-hybrid Cross Generalization Laws of probability indicate a 9:3:3:1 phenotypic ratio of F2 offspring resulting in the following: 9/16 of the offspring are dominant for both traits 3/16 of the offspring are dominant for one trait and recessive for the other trait 3/16 of the offspring are dominant and recessive opposite of the previous proportions; and 1/16 of the offspring are recessive for both traits.
Linkage and Gene Maps When genes are one separate chromosomes, they independently assort. If on the same chromosome, they will rarely separate and be inherited together (gene linkage) Actually it is the chromosomes that assort independently, not the genes. Mendel was just lucky with the genes he was looking at!
Crossing over in meiosis often separates linked genes. The distance between the two genes on the same chromosome are from each other affects the frequency of separation from each other during crossing-over. Further Apart Closer together
The frequency of crossing over between genes is actually an indicator of how far apart different genes are located from each other on the same chromosome. Use the frequency rates to make gene maps that show relative locations of genes with respect to each other.