Nov 4 Chapter 9 Patterns of Inheritance (continued)

 

Mendel’s Principles (continued)

 

monohybrid cross – Cross of parents differ in only one characteristics (traits)

 

Why did the white color trait disappeared in F1 generation and reappeared in F2 generation?

 

The following is Mendel’s explanation.

 

2)     Mendel’s principle of segregation (Mendel’s first law of genetics)

i.      Alleles-alternative forms of genes

eg. Purple allele, white allele

 

ii.    An organism has two alleles, one from each parent. 

The two alleles may identical or different.

 

iii.  Meiosis separates (segregates) two alleles. 

Each gamete contains only one allele of each gene.  Fertilization pairs two alleles once again.

 

iv. Dominant vs. Recessive

Dominant alleles can override traits from recessive alleles.

                 eg. Purple allele – dominant

                        white allele – recessive

Dominant alleles are represented by uppercase letters.

Recessive alleles are represented by lowercase letters.

                      eg. Purple allele – P

                            White allele – p

 

v.   Genotype vs. Phenotype

Genotype – genetic makeup

Phenotype – actual characteristics (traits)

          eg.  Purple flower is phenotype. 

                 Genotype of purple flower can be either PP or Pp.

 

                 White flower is phenotype.

                 Genotype of white flower is pp.

 

vi. Homozygote vs. Heterozygote

homozygous (n. homozygote) – two alleles are identical

          eg.  PP or pp

 

heterozygous (n. heterozygote) – two alleles are different

          eg. Pp

 

Recessive traits are expressed only when they are homozygous.

          eg. pp (homozygous) – white flower

 

Dominant traits are expressed when they are homozygous and heterozygous.

          e.g. PP (homozygous) – purple flower

                 Pp (heterozygous) – purple flower

 

Example 1 - Mendel’s monohybrid cross Fig 9.3 B, p158

 

Genotype of parental purple flower plant is PP.

Genotype of gametes produced by PP flower is ______.

 

Genotype of parental white flower plant is pp.

Genotype of gametes produced by pp flower is _______.

 

Fertilization of these gametes results in F1 generation of ______ genotype.  The characteristic (phenotype) of F1 generation plants is ________.

 

Genotypes of gametes produced by F1 generation is ____________.

 

Fertilization of F1 gametes

 

 

Punnett square – table showing possible combination of gametes.

 

Fertilization of F1 gametes results in

            Genotypic ratio           PP : Pp : pp = 1 : 2 : 1

            Phenotypic ratio        purple flower : white flower = 3 : 1

 

Mendel’s principle of segregation summary

1.   Individuals have two alleles for each gene.

2.   When gametes form by meiosis, the two alleles separate (segregate), each resulting gamete with only one allele of each gene.

3.   Fertilization pairs two alleles once again.

 

Expample2

In the Figure 9.2D, p157, round seed shape is dominant allele R and wrinkled seed shape is recessive allele r.  Predict the genotypic and phenotypic ratios of the F2 generation using Punnett square. 

3)     Dominant and recessive human traits Fig 9.8A, p163; table in p165

 

Dominant

Freckles                                             Widows peak

Free earlobe                                      Brown eyes

 

Dominant disorders

Huntinton’s disease 1/25,000

Dwarfism                                1/25,000

 

Recessive

No freckles                                         Straight hair line

Attached earlobe                             

 

Recessive disorders

Deafness

Cystic fibrosis                        1/1,800 Caucasians

Phenylketonuria (PKU)         1/10,000

Tay-Sachs disease               1/3,500 Jews

 

Genetics of autosomal dominant traits

-          They don’t skip generation.

-          If one parent is homozygous for a dominant trait, the probability of their child getting the same trait is ________.

 

 

-          If both parents are heterozygous for a dominant trait, the probability of their child getting the same trait is _______.

 

 

-          If one parent is heterozygous for a dominant trait and the other parent don’t have the trait, the probability of their child getting the same trait is _______.

 

 

Genetics of autosomal recessive traits

-          They can skip generation.

-          Carriers – Individuals who do not show the recessive traits, but carry recessive allele (gene).  They are heterozygous.

-          If one parent is heterozygous for a dominant trait and the other parent is homozygous for a recessive trait, the probability of their child getting the recessive trait is __________.

 

 

-          If both parents are homozygous for a recessive trait, the probability of their child getting the same trait is _______.

 

 

Work on these problems before the next lecture.  Make pedigrees (family trees) if necessary.

 

1.      Dwarfism is a dominant trait.  If one parent is dwarf and the other parent is normal height, the probability of their child being dwarf is ________ .

 

 

 

 

 

 

2.      Deafness is a recessive trait.  One parent is deaf and the other parent is normal.  If their first child is deaf, what is the probability of the next child being deaf.

 

 

 

 

 

 

3.      Cystic fibrosis is a recessive trait.  Neither parents have cystic fibrosis but their child has cystic fibrosis.  What are the genotypes of the parents?  Use lowercase letter c for the cystic fibrosis allele and uppercase letter C for normal allele.

 

 

 

 

 

 

What is the probability of the next child having cystic fibrosis?

 

 

 

 

 

 

4.      Huntington’s disease is a dominant trait.  One grandmother had Huntington’s disease but neither parents had Huntington’s disease.  What is the probability of the child having the disease?