How do traits get passed on from parent to offspring? Ch. 7 Levetin p. 103-112

1. Genetics is the field of biology that studies genes or inheritance.

Genes are segments of DNA that code for traits.

Example of genetics:

Bread wheat has been around for 9,000 years as seen by evidence from ancient Egypt.

20,000 varieties of bread wheat exist today. This has come about by mixing of traits through sexual reproduction.

2. Chromosomes

Chromosomes are strands of DNA. DNA is the genetic material. In humans, peas, corn and many organisms, chromosomes are found in pairs. Humans have 23 pairs of chromosomes. Corn has 20 chromosomes or 10 pair. Onions have 16 chromosomes or 8 pair.  The reason why chromosomes are found in pairs is organisms receive one copy of every chromosome from the male parent and one copy from the female parent.

Genes are segments along the chromosomes. Humans have about 30,000 genes. A very simple bacteria might have as few as 1,000 genes.

3. Karyotypes

A karyotype is a display of chromosomes. Human karyotypes are done to look for certain diseases. One common reason for a karyotype is an amniocentesis to look for Down syndrome, 3 copies of chromosome #21 instead of the normal 2.

Ploidy indicates the number of copies of each chromosome.  This course will focus on diploid organisms, those with 2 copies of every chromosome.  For example, humans, peas and corn are all diploid organisms having 2 copies of each chromosome.  Wheat plants, by comparison can have 4 copies (tetraploid) of each chromosome or even 6 copies (hexaploid) of each chromosome.  An example of  tetraploid wheat is durum wheat used to make the semolina flour for pasta.  The wheat used to make bread (Triticum aestivum) is  hexaploid (see p. 190 of Levetin).  This hexaploid wheat accounts for 90% of world production.

Mendel and Genes

1. Gregor Mendel is the father of genetics. He was an Austrian monk who lived in the 1800’s and who studied garden peas. He published his studies on peas in 1866, one year after Darwin’s "Origin of Species". However, due to the mathematical nature of his work and the fact that it was published in German, Darwin and his contemporaries never realized its significance. His work was rediscovered in about 1900 by scientists conducting similar experiments.

2. Mendel’s pea experimentsFig. 7.2 and 7.3, Table 7.1

Generation	Flower phenotype	Genotype
Parent 1 Parent 2	White Purple	pp PP
F1	Purple	Pp
F2	3 purple:1 white	1 PP:2Pp:1pp

Some terms:

Genes: segments of DNA

Alleles: alternate forms of a gene

Phenotype: physical characteristics that we see

Genotype: the actual alleles present

Example: If the gene for flower color is present in two forms and P=purple allele and p=white allele, then

PP=homozygous dominant

Pp=heterozygous

pp=homozygous recessive

-Solve the following problem

Widow’s peak (W) is dominant over continuous hairline (w). Bert and Ernestine marry knowing they are both heterozygous for Widow’s peak based on their parent’s phenotypes.

a. What is the phenotype of Bert and Ernestine?

b. What is the genotype of Bert and Ernestine?

c. What is their parent’s phenotype?

d. What phenotypic ratio is expected in the offspring of Bert and Ernestine?

e. What genotypic ratio is expected in the offspring of Bert and Ernestine?

Answers:

a. Widow’s peak

b. Ww

c. 1 Widow’s peak and 1 continuous hairline. This is the only way they could know they were heterozygous based on the parents phenotypes. They could also be heterozygous if both parent had Widow’s peak, but they would not know it.

d. 3 Widow’s: 1 continuous hairline

e. 1 WW:2 Ww:1 ww

3. Mendel’s principle of segregation

An individual has 2 copies of every gene but only 1 copy is passed on to the offspring

4. Mendel’s principle of dominance

If both dominant and recessive alleles are present, the phenotype will only show the dominant allele.

5. Testcross

Exercise:  Do Punnett squares of PPxpp  and Ppxpp.  Describe what the phenotypic outcome will be if the dominant phenotype parent is homozygous dominant vs. heterozygous.

6. Can you tell if a trait is dominant or recessive based on how common it is in a population? NO! NO! NO!

For example, Widow’s peak shows dominance over continuous hairline, yet in our class, 5 people had Widow’s peak and 20 continuous hairline. Another example: polydactyly, having an extra finger or toe is a dominant condition, yet, it is found in only 1/500 live births (usually it is just a boneless flap off the pinky, but can sometimes be completely functional).In our sample problems, we do all possible crosses so it seems like the dominant trait will always be more common. But in real life, mating is not random. Also, some traits may confer an advantage or disadvantage.

7. Recessive disorders

To have the disease, you must have two copies of the disease allele.

Example:

Cystic fibrosis is caused by a malfunctioning protein that transports chloride ions across the cell membrane. If T=Transport normal and t=cystic fibrosis than

TT=normal phenotype

Tt=normal phenotype but can pass the disease allele to an offspring (Carrier)

tt=cystic fibrosis

By analogy, think of the disease as being caused by a room with 2 doors. If one door is working, the second door does not matter. However, if both doors are broken then there is a problem.

Problem:

Two individuals who do not have cystic fibrosis (CF) have a child with CF.

a. What are the parents genotypes?

b. If the parents have another baby, what is the chance that baby will have CF?

c. If the parents have 2 more babies, what is the chance that both will have CF?

Answer:

a. Both parents must be carriers, Tt

b. ¼ or 25% from a Punnett square

c. 1/16 or 6.25% from the rule of multiplication

The rule of multiplication states that the probability of 2 (or more) independent events occurring is the product of their individual probabilities. For example, what is the chance of picking the Queen of Spades from a card deck? The chance of getting a queen is 1/13 and a spade ¼. Therefore, the chance of picking the queen of spades is 1/13*1/4 = 1/52.

To solve problem c, the chance of having 2 babies in a row with cystic fibrosis is ¼ * ¼ = 1/16.

In the past few years, a test has been developed to see if you are a carrier of CF and anyone planning to start a family should be tested.  You can read about the test for CF here.

Heterozygous advantage

Why don’t recessive disorders get eliminated due to natural selection?

Sometimes there is an advantage to the heterozygous genotype. For example, two alleles exist of the gene for hemoglobin, a protein in the red blood cells that carries oxygen.

If H=normal hemoglobin protein and h=sickle cell hemoglobin, then

HH=normal phenotype

Hh=carrier with normal phenotype

hh=sickle cell disease

In some parts of Africa, malaria is prevalent. Malaria is a disease caused by a protozoan that invades red blood cells. Learn more about malaria  and learn about the life cycle of Plasmodium.  Heterozygous individuals show resistance to malaria. This is called heterozygous advantage and explains why recessive genes often persist even in lethal diseases.

Genetic testing is available to see if you are heterozygous for some recessive disorders. For example, those of African ancestry can be tested as carriers for sickle cell. Learn more about sickle cell anemia. A test is now available to see if you are a carrier for cystic fibrosis and if you are thinking of having a baby, may wish to do this.

How can you lower the risk of having an offspring with a recessive disorder?

One way is to have offspring with an individual of a different ethnic background than yourself. This increases the likelihood that both of you carry different recessive diseases. The dominant genes that you carry will cover up the recessive genes of your partner and vice-verse. By the way, although hybrid vigor is a confirmed principle in biology, a sociologist would have a very different perspective on the advantages and difficulties of marriages between ethnic groups.

8. Dominant disorders

Some disorders are caused by dominant genes. Heterozygous individuals have the disorder. For example, in Huntington’s disease, a disease gene leads to accumulation of a toxic product which causes a nerve disorder in mid-life. Since individuals live past their child-bearing years, they pass it on to their offspring.

HH=Huntington’s disease

Hh=Huntington’s disease

hh=normal phenotype

Problem:

A man with Huntington’s marries a woman who does not have the disease. What are the odds their children will have the disease? Answer: ½

9. Genes vs. Environment (Nature vs. Nurture)

Not all diseases are 100% genetic. Many may be partially caused by genes.

Examples of diseases that are partially caused by genes and partially by environment:

Disease:

Alcoholism: although people may be predisposed to drink, social and culture influences will determine whether they use and abuse alcohol

Breast cancer, diabetes, heart disease: all these diseases have strong genetic components (i.e., they run in families) but eating a reduced fat diet will help reduce them

Some diseases are not caused by genetics at all. For example, colds are caused by a virus transmitted from person to person. 

If you want to learn more about genetics and disease, here a great website from the National Library of Medicine.  You can also find out more about what is known about a particular disorder by typing in at Online Inheritance in Man (OMIM).

10. Mendel’s Law of Independent Assortment

    a. Genes sort independently. If you get one gene, (Widow’s peak) it will not affect if you get a second gene (blue eyes)

    b. Explanation: During meiosis, chromosomes separate.

Which member of a chromosome pair goes to a given cell is random. Therefore, many combinations of chromosomes and genes from a parent can go to an offspring.

    c. Problem: Since 1/2500 white babies has cystic fibrosis, what is the probability of a white person being a carrier?

Answer: Let us start with the answer and work backwards.  Scientists believe 1/25 white people are carriers, heterozygous individuals.  The chance of two carriers marrying is

1/25 * 1/25 = 1/625  

 Since the chance of two heterozygous individuals having a homozygous recessive offspring is 1/4,  then 

1/625 * 1/4 = 1/2,500 and this is the actual number of babies with cystic fibrosis.

Beyond Mendelian Genetics  Levetin p. 108

1. Incomplete dominance

a. When true breeding red snapdragon flowers are crossed with true breeding white flowers, all flowers are pink!!!

b. Heterozygous individuals show a unique phenotype intermediate between the parents, a blending of traits.

c. Do the Punnet square crossing two pink flowers. What is your phenotypic and genotypic ratio?

Answer: Phenotypic ratio 1 red: 2 pink: 1 white

Genotypic ratio: 1 RR: 2Rr: 1 rr

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2. Multiple alleles

a. Think about blood types: A, B,AB, O. Both A and B are dominant. O is recessive

b. An individual carries only two copies of every gene

c. Within a population, there may be more than two alleles (forms) of that gene

3. Codominance

a. What about type AB blood? More than one allele can be observed phenotypically

b. Both A and B carbohydrates are found on the RBC

4. Polygenic inheritance

a. Height is not short or tall but has many intermediate values. Why?

b. One die=same amount of 1,2,3,4,5,6,

c. Two dice=few 2’s and 12’s, many 7’s

d. Polygenic inheritance: 2 or more genes affect a trait in an additive fashion

e. Explanation: Perhaps the gene for height produces growth hormone. The more hormone produced, the taller the individual. Because this is controlled by more than 1 gene, most people will get an intermediate level of hormone. This is a simple example of polygenic inheritance. The truth is height is more complex so don’t be tempted to inject growth hormone if you want to be taller

5. Chromosomal abnormalities

Numerous disorders are caused not by a single gene, but by chromosomal abnormalities that occur during meiosis. Meiosis is the process of cell division to form egg or sperm cells. Biology 100 will cover this process in detail, but it is beyond the scope of this course.

The most common chromosomal abnormality is Down syndrome, an extra chromosome #21. Women older than 40 are especially at risk for Down syndrome children.

Besides extra chromosomes, other abnormalities include deletion of part of the chromosome, duplication of part of the chromosome and assembly of two different chromosomes. Chromosomal abnormalities are quite serious since they involve many different genes. A course in genetics would discuss this in more detail.