Post on 30-Mar-2021
Genetics and Mendel February 21, 2012
Genetics and MendelGenetics • the study of heredity and variation • heredity the basic genetic blueprint of the parents is passed on to their offspring. • variation the genetic difference between parents and offspring
Genes distinct packets that pass on hereditary information from generation to generation.
• may not be expressed for several generations
Genetics and Mendel February 21, 2012
17th and 18th centuries • controlled experiments on inheritance were restricted • quick breeding and prolific animals (rats/mice) were not acceptable • plant were not thought of as “having sex” • religious restrictions on experiments and beliefs, yet some experimentation began
Genetics and Mendel February 21, 2012
1800's • thought that some information in the blood of both parents were mixed at conception (hence the term “bloodline”)
not based on careful scientific observation and experiment
1860's: Two notions of inheritance:
blending theory of inheritance “seeds” that controlled hereditary traits were blended together from generation to generation
pangenesis hereditary traits could be modified throughout a person's lifetime
Pangenesis theory of long giraffe neck
Genetics and Mendel February 21, 2012
Gregor Mendel (mid century) • Moravian Monk • demonstrated the basis of heredity used science and math to establish patterns in how traits were inherited • observed crosses in pea plants (Pisum sativum)
The pea plant was ideal for these studies: self pollinating so it was easy to control parental crosses easily grown, matured quickly, producing many seeds show several pairs of contrasting traits
Genetics and Mendel February 21, 2012
Controlling the pollination process
Genetics and Mendel February 21, 2012
Mendel's Experiments Phenotypes observable characteristics or traits (smooth or wrinkled peas, tall or dwarf height, purple or white flowers....) Viability different lines of plants could be expected to produce approximately the same number of plants for every 100 seeds planted P generation (P for Parental) cross between purebreeding plants that were different for only one contrasting pair of traits
Purebreeding Mendel began his experiments with plants that produced seeds that grew into plants identical to the parents for several generations
Genetics and Mendel February 21, 2012
Hybrids resulting plants from the cross between two purebreeding plants with only one contrasting pair of traits
Purebreeding tall plant Purebreeding dwarf plant
The F1 generation are the result of a tall and a dwarf parent cross. Although they are all tall, they have inherited contrasting height information from both parents and so must be a hybrid or combination.
Genetics and Mendel February 21, 2012
Purebreeding tall plant Purebreeding dwarf plant
F1 Hybrid (tall) F1 Hybrid (tall)
tall tall tall dwarf
Then the hybrid plants from the F1 generation were crossed.
The F2 generation that resulted from crossing the F1 hybrids revealed a 3:1 phenotypic ratio
Genetics and Mendel February 21, 2012
Purebreeding tall plant Purebreeding dwarf plant
F1 Hybrid (tall) F1 Hybrid (tall)
tall tall tall dwarf
The 3:1 ratio results from which of the original P generation parent traits is expressed or "turned on"
Dominant Trait expressed trait, use uppercase letters (eg. T = Tall)
Recessive Trait not expressed trait, use lowercase letters (eg. t = Dwarf)
T t
Label the plants with the trait they are expressing.
Genetics and Mendel February 21, 2012
Mendel’s Law of Segregation Factors units of inheritance (genes) for any given characteristic, there were several different forms of these genes called alleles
Ex: Gene for plant height has two different alleles, a dominant tall allele (T) and a recessive dwarf allele (t).
Homologous Chromosomes
Genetics and Mendel February 21, 2012
Purebreeding tall plant Purebreeding dwarf plant
F1 Hybrid (tall) F1 Hybrid (tall)
tall tall tall dwarf
Label the plants again, knowing that each plant carries two alleles for the height gene.
T t
Genetics and Mendel February 21, 2012
To illustrate a cross more graphically, a Punnet Square can be used. This works like a multiplication chart.
Consider the P generation:
Pure breeding tall plant (TT)
Pure breeding dwarf plant (tt) T
t Tt
can only give a T gamete
can only give a t gamete Only possible result is a hybrid for both alleles.Since T is dominant, all plants have a tall phenotype.
Genetics and Mendel February 21, 2012
The hybrids from the F1 generation are then crossed:
Hybrid (Tt)
Hybrid (Tt)
can give a T or a t gamete
can give a T or a t gamete
T t
T
t
T T T
T
t
t t t
Any F2 offsring with the T allele has a tall
phenotype
If no T allele is present, the dwarf allele t can be expressed
Genetics and Mendel February 21, 2012
Each plant's phenotype was determined by a pair of alleles that could be identical or different. • The allele expressed in the F1 is dominant (all hybrids were tall)• The allele not expressed in the F1 is recessive (the recessive dwarf allele could not be expressed until the F2 generation)
Law of Segregation: Members of a pair of alleles for a given trait are segregated (separated) when gametes are formed. A heterozygous plant that is Tt forms gametes that are T and t in equal numbers. The gametes are not a blend of the two traits.
Genotype gene makeup (the alleles an organism contains) Homozygous genotypes with identical alleles
(example: TT or tt) Heterozygous genotypes with different alleles
(example: Tt) How'd you like them peas?
Genetics and Mendel February 21, 2012
Try a cross for round peas R and wrinkled peas, r.
P
F1
F2 Phenotypic Ratio
Smooth Allele
Wrinkled Allele
R
rR
r
R
r
R r
Genetics and Mendel February 21, 2012
Mendel’s Law of Independent Assortment
Law of Independent Assortment: when two or more pairs of characteristics are considered at one time, each pair shows dominance and segregation independently of the other.
A plant that is heterozygous for two pairs of alleles,for example TtRr, (for height and pea shape) can form four types of gametes: TR, Tr, tR and tr.
These can be identified using the FOIL method from expanding binomials from math!
T t R r
Dihybrid an individual who is heterozygous for two traits: represented by the genotype AaBb for example. Dihybrid cross mating of two individuals both heterozygous for two particular traits (AaBb x AaBb).
Consider more than one trait in a cross:
Genetics and Mendel February 21, 2012
Smooth Allele
Wrinkled Allele
Tall Allele
Dwarf Allele
Try a dihybrid cross for height Tt pea shape Rr.
R
r
TR Tr t R t rT RT rt
t
R
r
gametes from F1 dihybrids T
t
F2 Phenotypic ratio tall
rounddwarf round
dwarfwrinkled
tallwrinkled
TtRr x TtRr
Genetics and Mendel February 21, 2012