Chapter 11 : Genetics
Genetics is the study of inheritance. First studied by Gregor Mendel in 1850-1870 using peas.
Mendels genetic factors proved in 1952 to be DNA.
DNA is divided into genes : each gene has the code for making one protein.
Human DNA has less than 30,000 genes.
Genes can be several alternative types called alleles example : allele B for brown hair, or b for blond.
You inherit two alleles ( one from each parent ).
A Dominant allele has an effect whether you have one copy of it, or two copies. eg BB or Bb ( brown hair ).
A Recessive allele only has an effect if you inherit two copies. eg bb (blond).
Genotype and Phenotype
Genotype is the genetic makeup of an individual eg Bb.
Phenotype is the physical appearance eg brown hair.
| Genotype | Phenotype |
|---|
| T T | Homozygous dominant | Tall plant |
| T t | Heterozygous | Tall plant |
| t t | Homozygous recessive | Short plant |
Mendel's Laws of Genetics
1) Different characteristics are caused by different alleles.
2) An organism inherits two alleles for each trait.
3) Each gamete only contains one allele for each trait.
4) In a heterozygous individual, only the dominant allele affects the trait.
Monohybrid cross - only one trait is studied ( eg black/brown fur Fig 11-6 ).
Dihybrid cross - two traits are studied ( black/brown fur and short/long hair Fig 11-8 ).
Mendel's results (don't memorize this table!)
| Trait | Dominant | Recessive |
|---|
| Flower color |
Purple | White |
| Flower position | Axial | Terminal |
| Seed color | Yellow | Green |
| Seed shape | Round | Wrinkled |
| Pod color | Green | Yellow |
| Plant height | Tall | Dwarf |
Test cross
A test cross is used to work out an unknown genotype ( dominant phenotype ).
The unknown is crossed with a homozygous recessive individual. Fig 11-7. If all the offspring have the dominant phenotype, the unknown parent must be homozygous dominant.
If some of the offspring have the recessive phenotype, the unknown parent must be heterozygous.
Dihybrid cross
Each trait is inherited independently. There are four possible phenotypes in the offspring. Fig 11-8.
If two heterozygous individuals are crossed ( eg BbSs : black, short-haired guinea pigs ) the result is a ratio of : 9 : 3 : 3 : 1
black, black brown brown
short hair long-hair short-hair long-hair
Probability
Punnett squares work out the probability of getting a particular offspring.
For example, if two heterozygous individuals breed, 3/4 of the offspring will have the dominant trait, and 1/4 will have the recessive trait.
In humans, Brown hair is dominant to blond, so if two heterozygous people have children, each child has a 1 in 4 (1/4) chance of being blond.
For a dihybrid cross, you multiply the probabilities together. For example: curly hair is dominant to straight, so two people who are heterozygous for both brown hair and curly hair could get the following offspring:
Brown/Curly Brown/Straight Blond/Curly Blond/Straight
3/4 x 3/4 3/4 x 1/4 1/4 x 3/4 1/4 x 1/4
9/16 3/16 3/16 1/16
Incomplete dominance
Both alleles affect the phenotype. Fig 11-18. So homozygous dominant (red) looks different from heterozygous (pink).
Both alleles have capital letters eg CR ( red ), CW (white).
If two heterozygous individuals are crossed the offspring are in the ratio :
1 : 2 : 1
homozygous heterozygous homozygous
dominant recessive
(red) (pink) (white)
Polygenic inheritance
Polygenic inheritance means trait is affected by several genes. So there is a range of variation in the population. Fig 11-22. The offspring of heterozygous parents can be very different from either parent
eg average sized parents can have a very tall child.
A graph of the expected offspring from heterozygous parents follows a normal distribution curve (bell curve). Fig 11-22.
examples : skin color, height.
Chromosomes
Genes are located on chromosomes; different species have different numbers of chromosomes, but about the same amount of DNA.
Human & Mouse
A typical human chromosome has roughly 1000 genes on it.
Linked genes are on the same chromosome.
Linked genes tend to be inherited together.
Linked genes are separated by crossing over in Prophase I. Fig 11-12.
This gives different results from the Punnett square prediction.
A genetic map gives the location of genes, obtained by working out the frequency of crossing over. Fig 11-13. Only about 10% of a chromosome is genes, the rest has an unknown function ( sometimes called junk DNA, although at least some is probably involved in switching genes on and off. Junk?).
Gender
Human cells have 22 pairs of non-sex ( autosomal ) chromosomes, that are the same in men and women, and 1 pair of sex chromosomes. The sex chromosomes are XX in the female and XY in the male. Fig 11-15.
X chromosomes are large (over 1000 genes). Y chromosomes are small ( less than 80 genes)
Human Births: 52% male, 48% female because sperm carrying the Y chromosome are lighter and swim faster than ones with the X chromosome.
Baby gender guarantee.
Sex linked diseases
These diseases mainly affect men.
They are caused by recessive genes on the X chromosome. Women have 2 copies of X, men have only 1 copy. Fig 11-16.
| WOMEN | MEN |
|---|
| Genotype | Phenotype |
Genotype | Phenotype |
| XN XN | Normal |
XN Y | Normal |
| XN Xn | Carrier |
Xn Y | Disease |
| Xn Xn | Disease |
| |
Sex-linked diseases are usually passed on from a carrier female to her son. Examples: colorblindness ( 8 % men colorblind, 0.5% women ). Fig 11-16. Test
Hemophilia ( blood does not clot ). Queen Victoria was a carrier.
Royal disease
Barr bodies
Only one X chromosome is active in women, the other condenses to form the Barr body on the edge of the nucleus.
The genes in the Barr body are inactive. Heterozygous females can have different X chromosomes active in different parts of the body eg Calico cats. Fig 11-17. Barr bodies are used to test for gender in sporting events like the Olympics, to make sure that female competitors really are female, and not men taking female hormones!
Olympic sex tests
Genomic imprinting
Some genes have different effects depending on whether they are inherited from the mother or the father.
These genes are imprinted, so some of the fathers genes are switched off in the sperm, and some of the mothers genes are switched off in the egg. If two sperm, or two eggs join, the embryo will not survive.
In mice, embryos with mainly the fathers genes have small heads, and large bodies.
Embryos with mainly the mothers genes have small bodies and large heads.
In mice, male imprinted genes affect lower parts of the brain ( feeding, reproduction, emotions ) whereas female imprinted genes affect the higher brain ( thinking, memory).
In humans, male imprinted genes increase the size of babies, female imprinted genes reduce the size.
Last edited October 2009 by David Byres, dbyres@fscj.edu