Genetics and Inheritance

The Scientific Study of Genes and the Inheritance of Traits

• Gregor Mendel (1860) = “Father of Genetics”

Experimented with pea plants Used self-fertilization (asexual reproduction)

and cross-fertilization methods (sexual reproduction)

chose simple traits to follow (flower color, height, seed color, seed texture etc.)

Key Terms To Know:

• Trait = any characteristic that can be passed from parents to their offspring

• Gene = genetic material on a chromosome that contains the instructions for creating a particular trait

• Allele = one of several varieties of a gene, an alternate form of the same gene for a given trait

example: A or a (same letter, different case)• Locus = the location on a chromosome where a

gene is located

Homologous Pairs

Every somatic (body) cell contains two copies of eachchromosome, one from each parent. This is called aHomologous Pair. Each chromosome in the pair contains agene for the same trait at exactly the same loci.

Monohybrid Crosses

• Monohybrid Cross = a cross involving only a single allele responsible for a trait; each parent contributes information for the trait (ex. Aa X AA)

Trait Characteristics

• Dominant trait = the trait that gets expressed in a monohybrid cross where the mode of inheritance is Complete Dominance; shown by a capital letter : A

• Recessive trait = the trait that is hidden in a monohybrid cross where the mode of inheritance is Complete Dominance; shown by a lower case letter : a

Allele Pairing

• Homozygous = the two alleles for a trait that an organism has are identical ex: Homozygous Dominant = AA , Homozygous Recessive = aa

• Heterozygous = the two alleles coding for a trait are different ex: Aa

Genotype vs. Phenotype

• Genotype = the actual genetic information, all the alleles that an organism possesses (the actual letters)

• Phenotype = physical appearance based on alleles

Mendel’s First Law of Heredity

• Law of DominanceLaw of Dominance: When a trait is Completely Dominant over another trait, then that dominant allele controls the gene pair. In the heterozygous form (Aa) the dominant trait only gets expressed. The recessive allele gets hidden.

Mendel’s Second Law of Heredity

• Law of SegregationLaw of Segregation: Pairs of alleles separate during the formation of the gametes (sex cells) during Meiosis I. Therefore each gamete will contain one allele for each gene.

Meiosis and Independent Assortment

Mendels Third Law Of Heredity

• Law of Independent AssortmentLaw of Independent Assortment : Gene pairs separate into the gametes both independently and randomly of each other.

• The rules of probability are used to describe how the different chromosomes and their alleles from parents assemble into gametes and ultimately into offspring.

PUNNETT SQUARES

• Used to track a trait from parents to offspring.• Shows possible ratios for offspring outcomes.

• P Generation = Parents

• F1 Generation = Offspring from the cross of the parents

• F2 Generation = Offspring produced from crosses among the F1

• F = filial, refers to sons and daughters

Monohybrid Cross

Monohybrid Cross ratios:Complete Dominance = 3:1 (Aa X Aa)

4:0 (AA X aa)2:2 (Aa X aa)

Punnett Square Rules

• 1. Make a key; show dominant and recessive alleles

• 2. Properly label parents genotypes, place them on the outside of the punnett square

• 3. Correctly perform the cross

• 4. Interpret the results; determine ratios

Test Crosses

– Suppose we have two Tall pea plants

• T= tall

• t = short

What are the possible genotypes for each plant?

• tt For A Short Plant

• TT OR Tt For a Tall Plant

To find out whether an individual showing adominant trait is homozygous or heterozygous,the individual can be crossed with an individualwhose genotype is known for certain.

Test Cross: cross the tall plant (TT or Tt) with a short

one (tt)

• If any recessive phenotypes show up in the next generation, the unknown genotype MUST be Heterozygous

• Homozygous tall x short TT x tt Heterozygous Tall x short Tt x

tt

x

All Offspring Would Be Tall!!

Either TT or Tt