Nov 8 Chapter 9 Patterns of Inheritance (continued)

 

 

3)  Mendel’s principle of independent assortment (Mendel’s second law of inheritance)

 

dihybridcross – Cross of parents differ in two traits.

 

“Each pair of alleles segregates independently during gamete formation.”

 

True for traits that resides in different chromosomes.

 

<Example>

Two traits in pea               Seed shape:              round (R), winkled (r)

                                           Seed colour:              yellow (Y), green (y)

Parents’ traits (genotype): round and yellow (RRYY) x  winkled and green (rryy)

Genotypes of gametes:               RY                                           ry

F1 generation (genotype):           All round and yellow ( RrYy)

 

Genotypes of gametes:               RY, Ry, rY or ry

 

F2 generation

 

Genotype ratio      RRYY : RrYY : RRYy : RrYy : RRyy : Rryy : rrYY : rrYy : rryy=

 

Phenotype ratio               

Round and Yellow: Round and Green : Winkled and Yellow : Winkled and Green =

 

Chromosome basis of inheritance

 

4)   Variation in Mendel’s principle

a.   incomplete dominance

-Heterozygotes exhibit intermediate phenotype.

example 1.  colour of snapdragon flower

red flower:      RR

white flower:   rr

pink flower:     Rr

                       

Monohybrid cross of red and white flower parents

P generation              RR x rr

F1 generation            Rr  (pink)

F2 generation

	R	r
R
r

genotypic ratio

phenotypic ratio:        red flower: pink flower: white flower =

 

example 2

hypercholesterolemia Fig 9.12, p168

b.   multiple alleles

-Many genes have more than two alleles in the population.

Example: ABO blood type

- Blood type is determined by presence and absence of specific sugar chains on the surface of red blood cells.

- Two types of sugar chains: A and B chains

- Alleles that express A and B chains are represented by I^A and I^B, respectively.

- The recessive allele i represents inability to produce neither A or B chains.

- A person may have                                                

 

Codominance – expression of two alleles in heterozygotes

(Note the difference between incomplete dominance and codominance.

incomplete dominance – intermediate phenotype in heterozygotes)

I^A and I^B alleles are codominant.

 

ABO blood type (continued)

-          Human bodies produce antibodies against molecules foreign to themselves. 

 

-          This limits types of blood a person can receive through blood transfusion.

-          Receiving wrong blood can be fatal!

-          Blood type can be used for paternity analysis.

 

c.   pleiotropic effects

A single gene can affect many characteristics.

example 1             sickle-cell disease (Fig 9.14, p170)

example 2             Hungtington’s disease

example 3             albino

 

d.   polygenic inheritance

-          A single characteristics can be influenced by multiple genes.

-          additive effects

ex.  height, skin colour

e.   inheritance of linked genes

i) linked genes

- Genes on the same chromosome tend to be inherited together. 

- They do not follow Mendel’s principle of independent assortment.

 

ex. sweet pea

Purple flower:        P,        red flower:      p

Long pollen:          L,         round pollen:  l

 

The P and L genes are on the same chromosomes.  If there is no crossing over, they will always segregate together.  P and L genes are linked.

 

dihybrid cross:      purple flower, long pollen                  x          red flower, round pollen

genotypes of parents:                  PPLL                          x                      ppll

 

genotypes of gametes:                PL                                                       pl

 

F1 generation:                              All PpLl (purple flower, long pollen)

 

genotypes of F1 gametes:          PL, pl

 

F2 generation

 

Genotypic ratio:

Phenotypic ratio:

 

ii) genetic recombination

In reality, F2 generation always contains a small proportion of recombinant phenotypes due to crossing over.

 

Crossing over produces new combinations of alleles (Fig 9.19A, p175).

In the example of sweet pea plants, crossing over would produce gamete genotypes such as Pl and pL.

 

As a result, about 10 % of F2 generation shows recombinant phenotype (Fig 9.18, p174).

 

Can you identify recombinant phenotypes in the Fig 9.18?

 

iii) recombination frequency vs. genetic map

Recombination frequency - the percentage of recombinants in offspring.

Recombination frequency (%) =

(number of recombinant / total number of offspring) x 100

 

ex.  Fig 9.18

            number of recombinant                    21 + 21= 42

            total number of offspring                   284+21+21+55= 381

            recombination frequency                  42 / 381 = 11 %

 

- Recombinant frequency is higher between genes located further apart in the chromosome.

- Physical distances between gene loci can be estimated via recombination frequency (Fig 9.20, p176) ® Genetic map

 

Example:

If recombination frequencies between three genetic loci A, B and C are

A and B          20 %

B and C          15 %

C and A          5 %

 

Genetic map of loci A, B and C would be:

The Nov 13 class will deal with some issues concerning the genetic testing.  Please read the following paragraphs before next class and think of what you would do if you were placed in these situations.  There is no right or wrong answers to these issues.  It is up to you to make decisions.

 

(These paragraphs are taken from “Your Genes, Your Choices.”  by Catherine Baker issued by American Association for the Advancement of Science (AAAS).  You can access the book from the web site http://ehrweb.aaas.org/ehr/books/ .)

 

Priya Should Find Out She Inherited a Fatal Disease (or should she?)

Priya has just lost her mother to an illness called Huntington’s disease. It was hard for Priya to watch her mother die. First her mother had strange changes of mood. Then her arms and legs began twitching. Soon she couldn’t talk or control her movements. In the end, she was totally bedridden and could barely get food down without choking. Priya knows that Huntington’s disease usually strikes people in middle age. It is always fatal, and there is no treatment. She also knows that since the disease is inherited, she has a strong chance of getting it herself. Priya just learned about a test she can take. The test will tell if she carries the gene for Huntington’s disease. She is tempted to take the test. She thinks that if she could find out once and for all whether she will get the disease, she could plan for her future. On the other hand, she wonders if it is better not knowing. At least then Priya would still have some hope. If you were Priya, what would you do?

 

Carlos and Mollie Can Have a Perfectly Healthy Baby (or can they?)

Carlos and Mollie want to have children. However, they haven’t tried to start a family yet because they disagree on something important. Carlos wants Mollie to get tested to see if she is a carrier for cystic fibrosis (CF). Mollie doesn’t want to do it. People with CF have mutations in one or more genes. These mutated genes give faulty instructions for the production of proteins that help move salt in the body. One result is that the lungs become clogged with mucus, making it hard to breathe. Another result is that the body has a hard time digesting food. The disease can be painful and lead to an early death. Carlos had a brother with CF. He hated seeing his brother suffer so much. His parents struggled with the hardship and expense of caring for a sick child who never made it to adulthood. Carlos doesn’t want to repeat that experience in his own life. That’s why he had himself tested for CF. Unfortunately, he found out that he is a carrier. CF is a recessive disorder. That means his children will have the disease only if they inherit the mutated gene from both parents. Mollie can get tested to see if she carries the CF mutation. If she does, then when she gets pregnant they can have the fetus tested to make sure it does not have two CF genes and is therefore free of the disease. Mollie would prefer simply not knowing what the risks are. She figures that once a baby is in their arms, they will be glad they had it, no matter what. If you were Mollie or Carlos, what would you do?

 

Howard’s Health Is Up to Him (or is it?)

Howard will turn 50 soon, and it worries him. His grandfather died of a heart attack in his fifties, and so did his father and uncle. Several years ago, a doctor told Howard that he was at high risk for heart disease because of his family history. But the doctor said that Howard could improve his chances if he lost some weight, stopped smoking, and exercised. The doctor also told Howard to come back every year for a checkup. Howard hasn’t gone on a diet, and he hasn’t given up his cigarettes or taken up exercise. He also hasn’t been back to the doctor. He’s afraid of what the doctor might find. Howard can’t make up his mind. Sometimes he thinks he should try to take better care of his health. Other times, he thinks that he should just accept the fact that he won’t live much longer and should get as much fun out of life while he can. If you were Howard, what would you do?