Genetics and the Work of Gregor Mendel
Gregor Mendel
Modern genetics began in the mid-1800’s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas Used experimental design Used mathematical analysis
Collected and counted peas Scientific method
Where are genes located?
On chromosomes! Remember
chromosomes are made up of DNA located inside of the nucleus
Every individual receives one copy of each gene from both parents
Mendel’s Work
Bred pea plants Cross-pollinated Raised seed and then
observed traits Allowed offspring to self-
pollinate and observed next generation.
Mendel collected data for 7 traits
Generations…
P1= parent generation F1= first generation after the parent generation F2 = second generation after the F1 generation
(F1 hybrid is a term used in genetics and selective breeding. F1 stands for Filial 1, the first filial generation seeds/plants or animal offspring resulting from a cross mating of distinctly different parental types.)
Mendel’s Peas in a “nutshell”
P1 = TT (homozygous tall) x tt (homozygous short) Results= 100% Tt heterozygous tall plants (F1
generation) F2= Tt x Tt (he crossed two offspring from
the F1 generation) Results= 25% TT, 50% Tt, 25% tt (F2 generation)
Law of Segregation
When Mendel performed cross-pollination between a true-breeding yellow pod plant and a true-breeding green pod plant, he noticed that all of the resulting offspring, F1 generation, were green.
He then allowed all of the green F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 3:1 ratio in pod color. About 3/4 of the F2 plants had green pods and about 1/4 had yellow pods. From these experiments Mendel formulated what is now known as Mendel's law of segregation.
Law of Segregation
Mendel's law of segregation states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.
Egg
T t
Sperm
The resulting union would be Tt.
Law of Segregation – four main ideas…
There are alternative forms for genes. For each characteristic or trait organisms inherit two
alternative forms of that gene, one from each parent. These alternative forms of a gene are called alleles. (such as T = tall, t= short)
When gametes (sex cells) are produced, allele pairs separate or segregate leaving them with a single allele for each trait.
When the two alleles of a pair are different, one is dominant and the other is recessive.
Law of Segregation
From Mendel's law of segregation we see that the alleles for a trait separate when gametes are formed (through a type of cell division called meiosis). These allele pairs are then randomly united at fertilization.
Traits come in these combinations: Homozygous= both letters are the SAME (TT, tt) Heterozygous = there is both a capital and lower case
letter in the pair (Tt)
Genotype vs. phenotype
Difference between how an organism “looks” and its genes Phenotype – what the organism looks like Genotype – genetic makeup of the organism
Explain cross using Mendel’s idea of
….Dominant and recessive
….Phenotype and genotype
What did Mendel’s Findings mean?
Some traits mask others Purple & white flowers are separate traits that do not
blend Purple x white ≠ light purple
Dominant allele – LAW OF DOMINANCE Functional protein that masks another gene
Affects the characteristic
Recessive allele Not noticable effect Allele makes a non-functioning protein
Warm UP
Cross a homozygous white flower with heterozygous white flower.
What is the probability of getting a purple flower?
Making Crosses
Alleles are represented as letters flower color alleles P (dominant) or p (recessive) True breeding purple flowers = PP True breeding white flowers = pp
PP x pp
Pp
Mendel’s system is just TOO easy!
Peas are genetically simple Most traits are controlled by a single gene Each gene only has 2 versions
1 completely dominant 1 recessive
Many genes, one trait
Polygenic inheritance Additive effects of many genes Humans
Skin color Height Weight Eye color Intelligence behaviors
Independent Assortment
When more than one gene passes from generation to generation, each gene separates independently of the other
Produces two factor crosses Yellow round peas x Green wrinkled peas RRYY x rryy
Human Skin Color AaBbCc x AaBbCc Can produce a wide range of shades Most children = intermediate skin color Some can be very light, some can be very
dark
Human Skin Color
Two Factor cross = dyhibrid cross
Coat color in other animals
Two genes: E, e and B, b Color = E; no color = e How dark the color is: B = black, b= brown
Incomplete dominance
Hybrids have an “in-between” appearance RR = red flowers rr = white flowers Rr = pink flowers
Make 50 % less color
Incomplete dominance
Codominance
Equal dominance Human blood ABO blood groups 3 versions
A, B, i A & B alleles are codominant Both A & B alleles are dominant over the i allele
The genes code for different sugars on the surface of red blood cells “Name tag” of the red blood cell
A type A female would like to have a child with a Type B male. The type A female’s genotype is IAi. The type B male is IBIB. What are the possible genotypes and phenotypes for their child?
What if the parents are Type O and Type AB? What are the possible genotypes and phenotypes for their child?
Many genes, one trait
Polygenic inheritance Additive effects of many genes Humans
Skin color Height Weight Eye color Intelligence behaviors
Independent Assortment
When more than one gene passes from generation to generation, each gene separates independently of the other
Produces two factor crosses Yellow round peas x Green wrinkled peas RRYY x rryy
Human Skin Color AaBbCc x AaBbCc Can produce a wide range of shades Most children = intermediate skin color Some can be very light, some can be very
dark
Human Skin Color
Two Factor cross = dyhibrid cross
Coat color in other animals
Two genes: E, e and B, b Color = E; no color = e How dark the color is: B = black, b= brown
Environmental Effect
Phenotype is controlled by both the environment and genes
altering the pH of the soil = Blue flowers appear when the soil has an acidic pH of 5.5 or lower
Coat color in Arctic animals is influenced by the environment.