Chapter 11 Notes

Mendelian Genetics

 

What is genetics?  The study of heredity, how characteristics are passed from one generation to another.

 

Mendel’s experiment

 

·    Gregor Mendel studied garden peas. Pea plants are usually self-pollinating. This means that the pollen (sperm) of one plant will fertilize the egg of the same plant. The seeds produced will inherit all of their characteristics from the single plant that bore them.

 

·    These self-pollinating pea plants produced true-breeding or pure offspring. They would produce offspring identical to themselves.

 

·    Mendel identified seven traits in pea plants.

 

·    Mendel wanted to see what would happen if he cross-pollinated two true-breeding plants with contrasting characteristics.

 

·    For example: Mendel wanted to see what would be the offspring of a tall plant X a short (dwarf) plant.

 

 

 

TT (Tall)  x tt (Short)

 

·    This is called a monohybrid cross because it only looks at the inheritance of one trait at a time.

 

·    Mendel expected to find all the offspring to be of medium height (a blending of tall and short plants.)

 

·    What Mendel found was that all the offspring were tall.

 

·    Mendel concluded that we have two factors (called genes) that control each trait. 

 

·    Each of these genes can be expressed in different forms (alleles).  ie) tall and short

 

·    Mendel also concluded that parents pass on at random only one allele for each trait to the offspring in their gametes. 

 

·    This is called The Principle of Segregation.

 

Mendel’s First Generation Cross

 

TT  x tt

 

 

T

T

 

t

Tt

 

Tt

t

Tt

Tt

 

 

Results of the First Generation Cross

Phenotype (physical appearance): all tall

Genotype (genetic combination): all Tt (hybrids)

 

·    Mendel next wanted to see if the recessive allele had just disappeared.

 

·    He allowed the hybrid offspring of the first cross to self-pollinate.

 

Mendel’s Second Generation Cross

 

Tt  x Tt

 

 

T

t

 

T

TT

 

Tt

t

Tt

tt

 

 

      Mendel expected to find all the offspring to be tall.

 

      What Mendel found was that 3/4 the offspring were tall and ¼ of the offspring were short.

 

Results of the Second Generation Cross

Phenotypic ratio: 3 tall: 1 short

Genotypic ratio: 1 TT: 2 Tt: 1 tt

 

The Principle of Dominance:

      Of the two alleles, one will be dominant over the other (recessive).

      Dominant traits are expressed even if only one of the alleles is dominant. 

      Recessive traits are expressed only if both of the alleles are recessive.

 

NOTE: Dominant alleles are usually represented by capital letters (T) using the first letter of the dominant trait, recessive alleles are represented by lower case of the same letter (t).

      We now call true-breeding plants homozygous: when the two alleles of a trait are the same.  ie) TT or tt

 

      We now call hybrid plants heterozygous: when the two alleles of a trait are different. ie)  Tt

 

 

 

 

 

 

 

 

 

 

 

 

 

Practice with Punnett Squares

Ex. #1 In humans, having freckles is dominant to not having freckles. Predict the genotypic and phenotypic ratios of a cross between a man heterozygous for freckles and a woman without freckles.

 

Ff  x ff

 

 

F

f

 

f

Ff

 

ff

f

Ff

ff

 

Phenotypic ratio: 2 freckles: 2 no freckles

Genotypic ratio: 0 FF: 2 Ff: 2 ff

 

 

 

Practical applications of monohybrid crosses are to determine if an organism showing the dominant phenotype is homozygous or heterozygous.

 

·    This is called a test cross.

 

·    Purpose: to determine if a dominant genotype is homozygous or heterozygous dominant.

·    The individual is crossed to a recessive.

·    If any of the offspring have a recessive phenotype, the genotype is heterozygous.

 

 

 

Ex: In dogs, normal height is dominant to dwarf height. A breeder wants to know if his normal dog carries a recessive gene. Determine the phenotypic ratios for his test cross.

 

Genotype of Normal Dog: _NN__ or _Nn___

Genotype of Dwarf Dog: __nn__

 

NN  x nn

 

 

N

N

 

n

Nn

 

Nn

n

Nn

Nn

 

Phenotypic ratio: 4 normal: 0 dwarf

Genotypic ratio: 0 NN: 4 Nn: 0 nn

 

 

Nn  x nn

 

 

N

n

 

n

Nn

 

nn

n

Nn

nn

 

 

Phenotypic ratio: 2 normal: 2 dwarf

Genotypic ratio: 0 NN: 2Nn: 2 nn

 

If any of the offspring show the recessive trait, then the original dominant parent must have been heterozygous.