LECTURE: MENDELIAN GENETICS
LECTURE: MENDELIAN GENETICS KIRSTIN BROWN
Lecture: MENDELIAN GENETICS Outline:
1. Theories of inheritance 2. Mendelian genetics 3. Law of segregation 4. Law of independent assortment
Theories of Inheritance Blending Inheritance o Blending inheritance: traits in offspring are an average of those of the parents
Morris – Biology, How life works. 2 nd edition.
o Does not explain the “reappearance” of a trait several generations after it “disappeared” o Incompatible with Darwin’s theory of evolution by natural selection
Mendelian Genetics Gregor Mendel o 1822 – 1844 o Determined laws of inheritance by hybridization experiments using garden peas o Followed a small number of easily recognizable traits
Morris – Biology, How life works. 2 nd edition.
Mendelian Genetics Gregor Mendel o Used true-breeding strains o Example: plants with purple flowers (when crossed with each other) only produce plants with purple flowers
Mendelian Genetics Gregor Mendel o Mendel’s objective: cross true-breeding strains and cross their progeny to identify any statistical patterns in the frequency of seven traits o His approach differed from other scientists at the time in 3 important ways: 1. Use of true-breeding strains 2. Focused on just one or a few traits at a time 3. Counted progeny and looked for patterns
Morris – Biology, How life works. 2 nd edition.
Mendelian Genetics Terminology o P1 generation: parental generation o F1 generation: offspring from P1 generation
o F2 generation: The F1 generation self-fertilizes
Mendelian Genetics Gregor Mendel o When Mendel crossed two true-breeding strains, one producing yellow seeds and one producing green seeds, he found that all progeny had yellow seeds o The trait that appears in the F1 generation is said to be dominant, while the other trait is recessive
o These traits are due to different alleles of the same gene
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o The F1 progeny in Mendel’s experiments crossing parental strains with different traits were not true breeding
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Each cell in the pea plant contains two alleles of each gene o In true-breeding strains, the alleles are identical and are said to be homozygous o When gametes form, the two alleles segregate half the gametes get one allele, while the other half get the other allele o In homozygous plants, all gametes have the same allele o When two gametes carrying different alleles fuse, the resulting zygote contains two different copies of the allele and is said to be heterozygous o These progeny then form gametes and the alleles again segregate
Law of Segregation Gametes receive only one allele for each gene o The progeny can be determined using a Punnett square
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Note that the yellow seeds of the F2 generation have the same phenotype but different genotypes
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o How can we tell if a yellow seed is AA or Aa? o Allow them to self-fertilize:
Law of Segregation Gametes receive only one allele for each gene o How can we tell if a yellow seed is AA or Aa? o Allow them to self-fertilize
o OR use a testcross: cross with a true-breeding recessive strain
Law of Segregation Gametes receive only one allele for each gene o Segregation of alleles occurs during meiosis:
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Not all traits follow the Mendelian rule of dominance o Incomplete dominance: phenotype of heterozygote is the average of the homozygote
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Not all traits follow the Mendelian rule of dominance o Incomplete dominance: phenotype of heterozygote is the average of the homozygote o Co-dominance: Both traits are expressed
Law of independent assortment Different genes sort independently of one another o Genes located on different chromosomes will segregate independent of each other during gamete formation
Law of independent assortment Different genes sort independently of one another o Genes located on different chromosomes (or far apart on same chromosome) will segregate independent of each other during gamete formation
Morris – Biology, How life works. 2 nd edition.
Practice Question
Consider a cross between two true-breeding fish strains. The female has stripes (dominant) and a short tail fin. The male is spotted with a long tail fin (dominant). All F1 progeny are striped with long tails. Use S and s to denote body pattern alleles and T and t to denote tail fin alleles. What are the genotypes of the parents?
What is the genotype of the F1 generation?
Lecture: MENDELIAN GENETICS Outline:
1. Theories of inheritance 2. Mendelian genetics 3. Law of segregation 4. Law of independent assortment
Theories of Inheritance Blending Inheritance o Blending inheritance: traits in offspring are an average of those of the parents
Morris – Biology, How life works. 2 nd edition.
o Does not explain the “reappearance” of a trait several generations after it “disappeared” o Incompatible with Darwin’s theory of evolution by natural selection
Mendelian Genetics Gregor Mendel o 1822 – 1844 o Determined laws of inheritance by hybridization experiments using garden peas o Followed a small number of easily recognizable traits
Morris – Biology, How life works. 2 nd edition.
Mendelian Genetics Gregor Mendel o Used true-breeding strains o Example: plants with purple flowers (when crossed with each other) only produce plants with purple flowers
Mendelian Genetics Gregor Mendel o Mendel’s objective: cross true-breeding strains and cross their progeny to identify any statistical patterns in the frequency of seven traits o His approach differed from other scientists at the time in 3 important ways: 1. Use of true-breeding strains 2. Focused on just one or a few traits at a time 3. Counted progeny and looked for patterns
Morris – Biology, How life works. 2 nd edition.
Mendelian Genetics Terminology o P1 generation: parental generation o F1 generation: offspring from P1 generation
o F2 generation: The F1 generation self-fertilizes
Mendelian Genetics Gregor Mendel o When Mendel crossed two true-breeding strains, one producing yellow seeds and one producing green seeds, he found that all progeny had yellow seeds o The trait that appears in the F1 generation is said to be dominant, while the other trait is recessive
o These traits are due to different alleles of the same gene
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o The F1 progeny in Mendel’s experiments crossing parental strains with different traits were not true breeding
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Each cell in the pea plant contains two alleles of each gene o In true-breeding strains, the alleles are identical and are said to be homozygous o When gametes form, the two alleles segregate half the gametes get one allele, while the other half get the other allele o In homozygous plants, all gametes have the same allele o When two gametes carrying different alleles fuse, the resulting zygote contains two different copies of the allele and is said to be heterozygous o These progeny then form gametes and the alleles again segregate
Law of Segregation Gametes receive only one allele for each gene o The progeny can be determined using a Punnett square
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Note that the yellow seeds of the F2 generation have the same phenotype but different genotypes
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o How can we tell if a yellow seed is AA or Aa? o Allow them to self-fertilize:
Law of Segregation Gametes receive only one allele for each gene o How can we tell if a yellow seed is AA or Aa? o Allow them to self-fertilize
o OR use a testcross: cross with a true-breeding recessive strain
Law of Segregation Gametes receive only one allele for each gene o Segregation of alleles occurs during meiosis:
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Not all traits follow the Mendelian rule of dominance o Incomplete dominance: phenotype of heterozygote is the average of the homozygote
Morris – Biology, How life works. 2 nd edition.
Law of Segregation Gametes receive only one allele for each gene o Not all traits follow the Mendelian rule of dominance o Incomplete dominance: phenotype of heterozygote is the average of the homozygote o Co-dominance: Both traits are expressed
Law of independent assortment Different genes sort independently of one another o Genes located on different chromosomes will segregate independent of each other during gamete formation
Law of independent assortment Different genes sort independently of one another o Genes located on different chromosomes (or far apart on same chromosome) will segregate independent of each other during gamete formation
Morris – Biology, How life works. 2 nd edition.
Practice Question
Consider a cross between two true-breeding fish strains. The female has stripes (dominant) and a short tail fin. The male is spotted with a long tail fin (dominant). All F1 progeny are striped with long tails. Use S and s to denote body pattern alleles and T and t to denote tail fin alleles. What are the genotypes of the parents?
What is the genotype of the F1 generation?
