6.2 Principles of Inheritance

You’ll be able to: To explain the chromosome theory of heredity To summarize Mendel’s 3 laws

Walter S. Sutton Born 1877, Utica, N.Y., U.S. geneticist 1902 Demonstrated Gregor Mendel’s concept of heredity. In 1903 concluded that chromosomes contain units of heredity

Chromosomal Theory of Heredity Inheritance of traits is controlled by genes Genes are located on chromosome Each color represents a different gene

Representing Alleles Designated by capital and lower cases pairs Capital letters = dominant allele Lowercase letters = recessive alleles A = dominant a = recessive

Genotype vs. Phenotype Genotype Describes the genes inherited by an organism. Represented by two letters Examples AA, Aa, aa Phenotype An individual's anatomical structure, physiology and behavior. Refers to everything that can be easily observed and measured about an plant, animal or human being.

Homozygous vs. heterozygous Same alleles Purebred Genotypes: AA or aa Heterozygous Different alleles Hybrid Genotypes: Aa

Genotype Symbol Genotype Vocab Phenotype TT homozygous DOMINANT or pure tall tall Tt heterozygous or hybrid tt homozygous RECESSIVE  or pure short short

Mendel’s 3 Laws the Law of Dominance the Law of Segregation the Law of Independent Assortment

Law of Dominance In a cross of parents that are pure for contrasting traits only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

Parent Pea Plants F1 Pea Plants (Purebred parents with opposite traits) F1 Pea Plants (their offspring) tall stem x short stem all tall stems yellow seeds x green seeds all yellow seeds green pea pods x yellow pea pods all green pea pods round seeds x wrinkled seeds all round seeds axial flowers x terminal flowers all axial flowers     All offspring express the dominant allele in their phenotype!!!!!

Punnett Square Proof Parents (P): TT x tt T = the dominant allele for tall stems t = recessive allele for short stems Parent Pea Plants  (P Generation) Offspring  (F1 Generation) Genotypes: TT x tt Phenotypes: tall x short Genotypes: 100% Tt Phenotypes: 100% tall

Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Example When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait. In real life this happens during a process of cell division called "MEIOSIS".  Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm.. 

If you cross two hybrid: 3/4 boxes will produce an organism with the dominant trait ("TT", "Tt", & "Tt") 1/4 boxes ends up homozygous recessive ("tt"). Parent Pea Plants  (Two Members of F1 Generation) Offspring  (F2 Generation) Genotypes: Tt x Tt Phenotypes: tall x tall Genotypes: 25% TT 50% Tt 25% tt 75% tall 25% short

Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

The genotypes of our parent pea plants will be: RrGg x RrGg "R" = dominant allele for round seeds "r" = recessive allele for wrinkled seeds "G" = dominant allele for green pods “g" = recessive allele for yellow pods

RrGg x RrGg Notice that we are dealing with two different traits: (1) seed texture (round or wrinkled) (2) pod color (green or yellow).  Notice also that each parent is hybrid for each trait (one dominant & one recessive allele for each trait).

We need to "split" the genotype letters & come up with the possible gametes for each parent.  Keep in mind that a gamete (sex cell) should get half as many total letters (alleles) as the parent and only one of each letter. So each gamete should have one "are" and one "gee" for a total of two letters. 

There are four possible letter combinations: RG, Rg, rG, and rg. These gametes are going "outside" the p-square, above 4 columns & in front of 4 rows.  We fill things in just like before --- "letters from the left, letters from the top". When we finish each box gets four letters total (two "are's" & two "gees").

The results from a dihybrid cross are always the same:   RG Rg rG rg RRGG round RRGg round RrGG round RrGg round RRgg round Rrgg round rrGG wrinkled rrGr wrinkled rrGg wrinkled rrgg wrinkled The results from a dihybrid cross are always the same: 9/16 boxes show dominant phenotype for both traits (round & green) 3/16 show dominant phenotype for first trait & recessive for second (round & yellow) 3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green) 1/16 show recessive form of both traits (wrinkled & yellow).

Results a green pod can have round or wrinkled seeds a yellow pod can have round or wrinkled seeds The different traits do not influence the inheritance of each other.  They are inherited INDEPENDENTLY.

INDEPENDENT ASSORTMENT Summary LAW PARENT CROSS OFFSPRING DOMINANCE TT x tt tall x short 100% Tt tall SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods