Formal and easy notes on GENETICS (1.Genetic Crosses. 2.Dominant and Recessive Genes. 3.Incomplete Dominance. 4.Pedigree Studies. 5.Sex Determination. 6.Gregor Mendel. 7.Mendel’s 1st Law. 8.Mendel’s 2nd Law. 9.Summary of Mendel’s Laws. 10.Gene Linkage. 11.Sex Linkage.)

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Topics.

• 1.Genetic Crosses.
• 2.Dominant and Recessive Genes.
• 3.Incomplete Dominance.
• 4.Pedigree Studies. 
• 5.Sex Determination.
• 6.Gregor Mendel.
• 7.Mendel’s 1^st Law. 
• 8.Mendel’s 2^nd Law.
• 9.Summary of Mendel’s Laws.
• 10.Gene Linkage.
• 11.Sex Linkage. 

Genetics is the science of heredity and how new life changes and varies in their characteristics. Sexual reproduction in humans involves 2 gametes; one male gamete, the sperm, and one female gamete, the egg.

As was discussed in the cell division webpage the gametes are haploid (n). When fertilisation occurs the resulting fertilised egg is diploid (2n) Genetic variations occur as a result of this union.

As was discussed in the heredity webpage the physical characteristics of organisms are developed as the protein builds up their bodies. These proteins are formed as a result of genes carried on chromosomes.

In genetics genes are represented by letters. There are usually 2 different types of the same gene; one is dominant and one is recessive. An example of this is: T is a gene for tall and t is a gene for short. The two versions of the same gene are called alleles. The two alleles are formed at the same position, or locus, on the chromosome.

REMEMBER: THE 2 ALLELES ARE GOTTEN FROM THE 2 GAMETES OF SEXUAL REPRODUCTION. THAT IS WHY THEY CAN DIFFER.

IN ASSEXUAL REPRODUCTION THE NEW LIFE IS ALWAYS IDENTICAL TO THE PARENT CELL BECAUSE THERE IS ONLY 1 PARENT.

Dominant genes will always prevent the recessive gene from working. If a person has 2 dominant alleles for tallness: T T then he will be tall. If a person has 1 dominant and I recessive allele for tallness: T t then he will be tall. The only way the recessive gene will be expressed is if he has 2 recessive alleles for short:    t t then he will be short.

If the pair of genes controlling the characteristic has identical alleles, TT or tt, we call it a homozygous pair of genes. If the pair of genes controlling the characteristic are different alleles, Tt or Cr, or Cw we call it a heterozygous pair of genes.

When we express the genotype of a characteristic we state the pair of alleles. Genotype examples are TT, Tt, tt, BB, Bb, bb, etc.

When we state the physical characteristic expressed by the genotype we are stating is phenotype. The phenotype for TT is tall while the phenotype for tt is short. These phenotypes could vary because of environment effects. This is especially true in terms of genotypes and phenotypes for intelligence. The upbringing, environment, and education experiences greatly affect the phenotype.

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GENETIC CROSSES

REMEMBER: THE 2 ALLELES ARE GOTTEN FROM THE 2 GAMETES OF SEXUAL REPRODUCTION. THAT IS WHY THEY CAN DIFFER.

When working on genetic crosses you must state the capital letter which represents the dominant allele and the small letter that represents the recessive allele.

The following is an example of how to work out genetic crosses:

A is a dominant characteristic.	a is a recessive characteristic.
This bird has two genes for red feathers.   Its genotype is AA.   Its phenotype is RED	This bird has two genes for blue feathers.   Its genotype is aa.   Its phenotype is BLUE

This Punnett Square shows how we can diagram the genes.

The orange bird has two dominant A genes. We put two A s along the top of the square.

The blue bird has two recessive a genes. We put two a s down along the left side of the square.

All the offspring have the genes Aa.

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Dominant and Recessive Genes

They will all have orange feathers (phenotype), but will carry a recessive gene for blue feathers (genotype).The progeny are the offspring produced.This is called the F1 generation progeny.

Now suppose that two individuals from the F1 generation become parents. Here they are!
The baby birds are called the F2 generation. You can see how their genes work out. The offspring are coded in the squares. One bird will be orange with two AA genes.
Two birds will be orange with genes coded Aa.
One bird will be blue and will have two recessive aa genes. Individual nests of birds may not turn out exactly like this, but if there are many baby birds, they will work out genetically with the ratios 1:2:1.

Genotype: 1 AA, 2 Aa, 1 aa

                                                  Phenotype: 3 orange feathers, 1 blue feathers

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Incomplete Dominance

Incomplete dominance is the situation where two different alleles are equally dominant. When this occurs the heterozygous genotype that is produced is an intermediate phenotype (blend) between the two respective homozygous genotypes. This is also called codominance.

In this example AA genotypes have red, Aa genotypes pink and aa genotypes whitish flowers. Note that the heterozygous genotype Aa is a blend of red and white.   

Note: This is the F1 generation.

 In the F2 generation two pink flowers will produce 2 pink phenotypes, 1 red phenotype, and 1 white phenotype.

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Pedigree Studies

          A pedigree is a diagram showing the genetic history of a group of related organisms.

A pedigree showing the occurrence of a recessive trait in three generations of a family

. 

Circles in a pedigree represent females and squares represent males. A horizontal line between a circle and a square indicates a marriage or partnership.Vertical lines indicate the children from the marriage or partnership. In this activity, a filled-in circle or square shows that the individual has both alleles for the trait. A half-filled-in circle or square indicates that the individual has one recessive allele for the trait.

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Sex Determination

Our cells have 46 chromosomes. There are 23 pairs. Remember, we get 23 from our mother and 23 from our father. We have 22 pairs of chromosomes calledautosomes. These are the chromosomes that control our body growth, enzymes, etc. We have 1 pair of sex chromosomes.

If the person is a male that pair is composed of an X and a Y chromosome. The male’s genotype is XY.

 If the person is a female she will have a sex chromosome composed of 2 Y chromosomes. The female’s genotype is XX.

  

The Punnett Square for the fertilisation of a sperm and an egg is:

If the child has a genotype of XX then it becomes a girl. If the child has a genotype of XY then it becomes a boy.

As you can see the ratio of males to females is 1:1.

         

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Gregor Mendel

Gregor Mendel studied 7 characteristics of pea plants. He studied:

As a result of Mendel’s work the study of genetics began. He discovered that, although an organism may have genotypes for 2 different physical traits (phenotypes) the organism will only exhibit one of those traits.

The work he did for the height of the plants is a follows:

He discovered although the parents of the first generation had TT and tt genotypes all the progeny of the F1 generation were tall.

He then discovered that in the F2 generation there was a 3:1 ratio between tall and short.

The Punnett Square for Mendel’s F1 generation is:

The Punnett Square for Mendel’s F2 generation is:

Note that in the F1 generation the parents were homozygous. One was TT and one was tt. All the progeny were tall because all the progeny had Tt genotypes with T being the dominant characteristic.

In the F2 generation a Tt plant was used to self-pollinate itself. All of the phenotypes in this combination were Tt. As a result the F2 progeny were 3 Tt and1 tt.

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Mendel’s Laws

First Law- The Law of Segregation

1.           This law states:

a.   In diploid organisms, chromosomes occur in matching pairs. These pairs are called homologous chromosomes. These are chromosomes that pair at meiosis, have the same length and banding pattern plus carrying the same genes at the same locus. They have the same sequence of genes.

Notice that these two chromosomes are homologous because they have alleles at the same position on the chromosome but one allele is for purple flowers and the other for white flowers.

b.   For each characteristic or trait organisms inherit two alternative forms of that gene, one from each parent. These alternative forms of a gene are called alleles.

c.    When gametes (sex cells) are produced, allele pairs separate or segregate leaving them with a single allele for each trait.

d.   When the two alleles of a pair are different, one is dominant and the other is recessive.

Example:

If your eyes are blue, green or grey you have two alleles for blue eyes (bb), then your gametes must have a blue allele (b); if your eyes are brown you might have two brown allele (BB), then your gametes have one allele for brown (B) or you might have one allele of each kind (Bb), in which case you make two kinds of gametes some contain the brown allele (B) and some contain the blue allele (b).

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Second Law- Law of Independent Assortment

Mendel’s Second Law involves dihybrid crosses. Dihybrid crossing involves the study of 2 characteristics at the same time.The Law of Independent Assortment states that alleles for different traits are distributed to sex cells (& offspring) independently of one another.

Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another.  In other words, being tall didn't automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the differenttraits seem to be inherited independently.

The genotypes of our parent pea plants will be:

RrGg x RrGg where
"R" = dominant allele for round seeds
"r" = recessive allele for wrinkled seeds
"G" = dominant allele for green pods
"g" = recessive allele for yellow pods

Notice that we are dealing with two different traits: (1) seed texture (round or wrinkled) & (2) pod color (green or yellow).  Notice also that each parent is hybrid for each trait (one dominant & one recessive allele for each trait).

	RG	Rg	rG	rg
RG	RRGG round	RRGg round	RrGG round	RrGg round
Rg	RRGg round	RRgg round	RrGg round	Rrgg round
rG	RrGG round	RrGg round	rrGG wrinkled	rrGr wrinkled
Rg	RrGg round	Rrgg round	rrGg wrinkled	rrgg wrinkled

We need to "split" the genotype letters & come up with the possible gametes for each parent.  Keep in mind that a gamete (sex cell) should get half as many total letters (alleles) as the parent and only one of each letter. So each gamete should have one "R or r" and one "G or g" for a total of two letters. There are four possible letter combinations: RG, Rg, rG, and rg. So, when the two parents’ gametes form a new organism the punnett square will look like this:
 

The results from a dihybrid cross are always the same:

9/16 boxes (offspring) show dominant phenotype for both traits (round & green),
3/16 show dominant phenotype for first trait & recessive for second (round & yellow),
3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green), &
1/16 show recessive form of both traits (wrinkled & yellow).

So, as you can see from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod.  The different traits do not influence the inheritance of each other.  They are inherited INDEPENDENTLY.

Interesting to note is that if you consider one trait at a time, we get "the usual" 3:1 ratio of a single hybrid cross (like we did for the Law of Segregation). For example, just compare the color trait in the offspring; 12 green & 4 yellow (3:1 dominant: recessive).  The same deal with the seed texture; 12 round & 4 wrinkled (3:1 ratio).  The traits are inherited INDEPENDENTLY of each other.

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SUMMARY OF MENDEL’S LAWS

LAW	PARENT CROSS	OFFSPRING
DOMINANCE	TT x tt tall x short	100% Tt tall
SEGREGATION	Tt x Tt tall x tall	75% tall 25% short
INDEPENDENT ASSORTMENT	RrGg x RrGg round & green x round & green	9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods

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Linkage (Gene Linkage)

Gene linkage occurs when traits for 2 separate characteristics occur on the same chromosome.

The characters Mendel examined happened to be on separate chromosomes. That is why he observed independent assortment. If, however, the genes are on the same chromosomes, they will be inherited together. For example, consider the following parental nuclei. Both father and mother have a pair of chromosomes with alleles for two different genes:

If we look at this with a punnet square what is going to happen in the next generation:

The phenotype ratios are still 3:1, but there are fewer genotype combinations than in the usual cross involving two alleles.

Remember: With independent assortment the phenotypes resulted in a 9:3:3:1 ratio.

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Sex-linked Genes or Sex linkage

        Genes or traits whose controlling genes are on the X sex chromosome but not on the Y sex chromosome. As a result recessive phenotype occurs more often in males than in females.

There is yet another, unrelated, special case that means something totally different, yet has a similar-sounding name. This is sex-linked genes, genes located on oneof the sex chromosomes (X or Y) but not the other. Since, typically the X chromosome is longer, it bears a lot of genes not found on the Y chromosome, and thus most sex-linked genes are X-linked genes. One example of a sex-linked gene is fruit fly eye colour. An X chromosome carrying a normal, dominant, red-eyed allele would be symbolized by a plain X, while the recessive, mutant, white-eyed allele would be symbolized by X' or X^w. A fly with genotype XX' would normally be a female with red eyes, yet be a carrier for the white-eyed allele. Because a male typically only has one X chromosome, he would normally be either XY and have normal, red eyes or X'Y and have white eyes. The only way a female with two X chromosomes could have white eyes is if she would get an X' allele from both parents making her X'X' genotype. The cross between a female carrier and a red-eyed male would look like this:

	X	Y
X	XX XY XX' X'Y
X'

Notice that while there is a “typical” ratio of ¾ red-eyed to ¼ white-eyed, all of the white-eyed flies are males.

Sex-linked traits act just like recessive ones except they also bow to the will of the sex of the child. Genes are carried on things called chromosomes. Most people already know that in humans the man has an X and Y chromosome and the female has two X chromosomes. This is the reason that only the man can determine the sex of the child. Women can only provide X chromosomes while a man can provide either. Now, the X chromosome is bigger and can carry more genetic information on it than the Y can.

This is where sex-linked traits come in. Because the X is bigger it means that some genes carried on it are not carried on the Y chromosome. These genes can be expressed even without a corresponding partner on an X chromosome. They also cannot be blocked out unless there is another X chromosome carrying a dominant partner.

In humans, two well-known X-linked traits are haemophilia and red-green colour-blindness. Haemophilia is the failure (lack of genetic code) to produce certain substance needed for proper blood-clotting, so a haemophiliac’s blood doesn’t clot, and (s)he could bleed to death from an injury that a normal person might not even notice.

One human sex-linked trait is Haemophilia. Haemophilia is a disease that keeps a person's blood from clotting when he is cut. Haemophiliacs can easily bleed to death and must be very careful not to injure themselves. Many also take daily injections to help the problem. Because haemophilia is a sex-linked disease most of the people who have it are men. Women can carry the gene for haemophilia but will not be affected by it because their second X chromosome will block it out with a healthy gene. They must have two copies of the defective gene to display the disease. Inheriting two copies is highly unlikely. Men carrying haemophilia do not have another X chromosome so they will have haemophilia with only one gene for it. Mothers carrying one gene for haemophilia take a great risk with having children because there is a 50% chance their sons will end up with the disease. Here's how it works:

As you can see, at least half of her children (boxes 1 and 2) will inherit the defective gene. One, a daughter, will only carry the gene. The other, a son, will have the disease haemophilia. The last two children (boxes 3 and 4) will carry healthy genes. Of course these are only the possibilities of what her children could end up with. She could very well end up giving it to all her children or none at all. It's just a matter of chance. Now let's take a look at what will happen if this woman's haemophiliac son has children with a healthy woman:

In this case half the children will still inherit the gene. All the sons will be safe but all the daughters will end up carrying haemophilia and could end up passing it on to their children. It is in this way that sex-linked genes can disappear and reappear from generation to generation.