GENETICS and HEREDITY

* Gregor Mendel and his pea plants experiments (1857-1865)

genetics: the branch of biology that studies the transmission of hereditary information from parents to offspring

WHY MENDEL? WHY PEAS?

Mendel was a monk who was educated--he knew MATH! He also had a lot of free time.
As for the peas? Well, they were:
* easy to grow
* easy to pollinate (either self or cross)
* easy to study their TRAITS (characteristics) such as: height, seed pod shape, seed color, seed pod color, seed texture,flower position, and seed shape

Mendel kept very accurate records for hundreds of individual plants. He used statistical analysis (MATH) to study the traits of different generations. Through his findings, he proposed that the traits were passed on by some kind of hereditary factors (now know to us as DNA).

Here's basically what he did:

One trait he studied was plant height. Mendel pollinated all tall plants for many generations to get a pure population of tall plants. He also pollinated all short plants to get a pure population of small plants. See picture below...

X means to cross
(as in cross-pollinate)

tall plants X tall plants as well as short plants X short plants

He then cross-pollinated a member of the tall pure population with a member of the short pure population...

P1 (pure parent generation)
P1 TALL PLANT X P1 SHORT PLANT

ALL TALL PLANTS
F1 (first filial [family] generation)

Where did the short trait go? Mendel then crossed two tall members of the F1 generation.

F1 TALL PLANT X F1 TALL PLANT
THIS IS CALLED A MONOHYBRID CROSS (hybrids known for one trait)

F2 (second filial generation)

For every 3 TALL PLANTS there was 1 SHORT PLANT

So, the short trait did not disappear. It was present in the F1 generation, but IT WAS NOT EXPRESSED (this means it did not show up!) Somehow, it was expressed (visible) in the F2 generation.

I. Mendel's Principal of Dominance
* because the tall trait showed up more than short, Mendel call this trait DOMINANT
* the short trait, because it seemed 'weaker' than the tall trait, was called RECESSIVE

After this discovery...well, um...NOBODY CARED! Later on, when scientists studies meiosis in Drosophila (genus name for fruit flies), they linked together Mendel's factors with the chromosomes in gametes.

* Mendel's factors are now called GENES
* genes are segments of chromosomes (DNA) that code for a characteristic
* these characteristics can be:

genes are lined up on chromosomes in a certain order, like beads on a string

* homologous (similar) chromosomes have the same order of genes...HOWEVER..
* these chromosomes might have the same forms of a gene

ex. trait=eye color can be blue, black, brown, hazel, green, etc.

* these different forms of a gene are called alleles

* in simple patterns of inheritance, there are 2 different forms of a gene (alleles), where one is dominant and one is recessive

* DOMINANT IS SHOWN BY CAPITAL LETTERS
* recessive is shown by lowercase letters

ex. in Mendel's peas T = tall t = short

* diploid (having 2 sets of chromosomes) organisms have 2 copies of genes, one on each chromosome they have

Notice that what the organism looks like and the kind of genes it has are two separate ways to classify an organism.

phenotype: what the appearance of an organism is (how it looks) ph = physical

genotype: what the genetic makeup of an organism is (what genes it has) gen = genes

recessive phenotypes can only be expressed (show up) when there are 2 copies of a recessive gene present at the same time...if a dominant gene was there, then that would be expressed, hiding the recessive gene!

II. Mendel's Principle of Segregation and Recombination
* when gametes are formed during meiosis (DIPLOID to MONOPLOID) the homologous chromosomes (which contain the many different genes) separate randomly
* this is called SEGREGATION

* if the organism has a homozygous (the same) genotype for a given trait (TT or tt), then all gametes will also have that trait

TT <----diploid----> tt
meiosis
T or T <----monoploid----> t or t

* if the organism has a heterozygous (not the same) genotype for a given trait (Tt), then half of the gametes will have one trait, and the other half will have the other trait

Tt <----diploid
meiosis
T or t <----monoploid

* when the monoploid gametes fuse during fertilization, the diploid number of chromosomes is restored
* this can result in a new combination of genes
* this is called RECOMBINATION

Let's look back at Mendel's first few pea plant crosses.

P1 (pure parent generation) cross was: TT X tt

F1 (first filial generation) cross was Tt X Tt

F2 (second filial generation) offspring had genotypes of TT, Tt, tT, and tt

Now, compare the different phenotypes (physical appearance) and genotypes (genetic makeup).

For the F2 generation what is the PHENOTYPIC RATIO?

3 tall : 1 short

For the F2 generation, what is the GENOTYPIC RATIO?

1 homozygous dominant : 2 heterozygous : 1 homozygous recessive

PHENOTYPE DOES NOT ALWAYS EQUAL GENOTYPE!

* to show segregation (separation of genes/chromosomes) and recombination (during fertilization), we use Punnett Squares

Punnett Square Method
Follow my steps to carry out a Punnett square problem...(unfortunately the steps are shown in detail in class. These notes will make more sense if you weren't absent. If you were, set up a time after school to learn it!)

Problem: A homozygous tall pea plant is crossed with a short pea plant. (Remember, tall is dominant over short for pea plants!) What would the expected genotypes (the genotypic ratio) and phenotypes (the phenotypic ratio) of their offspring?

STEP 1: set up the key that will show how each allele (form of a gene) will be shown

T = tall
t = short

STEP 2: set up the parents that will be crossed; use the information from the word problem itself

TT x tt
a tall homozygous parent a short parent

remember, to be short, it has to have both short alleles!

STEP 3: do the Punnett square; that is, put one parent on the top and the other parent on the left of the square...then from a blank box, take the allele on the top and the allele from the left and fill them in

STEP 4: show the results (ratios) of the cross

GENOTYPIC RATIO:

PHENOTYPIC RATIO:

TRY THESE PUNNETT SQUARE PROBLEMS
1) Use a Punnett square to show the offspring of a cross between two pea plants that are heterozygous for height (Tt). Give the phenotype and genotypes of the offspring. (That is, what is the phenotypic and genotypic ratios!)

2) Use a Punnett square to show the offspring of a cross between a pea plant that is homozygous tall and one that is heterozygous. Give the phenotypes and genotypes of the offspring.

3) Use a Punnett square to show the offspring of a cross between a pea plant that is heterozygous tall and one that is homozygous recessive for height. Give the phenotypes and genotypes of the offspring.

 

A) Incomplete Dominance:
* when one allele (form of a gene) is partially dominant over the other
* it is a blending of the alleles

ex. Four O'clock Flowers (key below)

More practice problems
4) Use a Punnett square to show the offspring of a cross between a red Four O'clock flower and one that is heterozygous. Give the phenotypes and genotypes of the offspring.

5) Use a Punnett square to show the offspring of a cross between a homozygous white Four O'clock flower and one that is heterozygous pink. Give the phenotypes and genotypes of the offspring.

ex. cattle coat color (key below)

CR = red
CW = white
CWCR or CRCW = roan (where both red and white are expressed

 

C) Multiple Alleles:
* when there are more than 2 alleles for a gene
* in human blood types there 3 possible alleles for the ABO blood typing system

IA = A antigen

IB = B antigen

i = no antigen

" both IA and IB are dominant over i but,
" both IA and IB are codominant with each other (that means, they are both expressed equally)

More practice problems
7) Four newborn babies in the delivery room of the hospital at the same time were mixed up by the nurse who attached the wristbands. The blood types of the four babies were known to be AB, O, A, and B. How did the doctors find out which baby belonged to which set of parents? Carry out all possible crosses to determine which baby belongs to which set of parents. (HINT: You may not have to use all the Punnett squares!)

Parents #1 had blood types O and AB

Blood type of baby: _______

Parents #2 had blood types AB and B

Blood type of baby: _______

Parents #3 had blood types O and O

Blood type of baby: _______

Parents #4 had blood types O and A

Blood type of baby: _______

 

8) If a heterozygous A person and a homozygous B person have children, can they have an A type baby?

D) Sex-Linked Genes:
* most diploid organisms have 2 different types of chromosomes

* humans have 22 pairs of autosomes and 1 pair of sex-chromosomes

XX = female XY = male

* women (XX) always pass an X chromosome; men can pass an X or a Y (a 50%-50% chance)
* this is determined during the random selection process during the production of gametes (meiosis)
*besides the sex characteristics, there are also other traits on these chromosomes

* because the Y chromosome is actually smaller than the X chromosome, men only have one copy of certain genes...
* so...if the X chromosome has a recessive allele, that will be expressed because that is all that is there! (key below)

More practice problems
9) Use a Punnett square to show the offspring of a cross between a woman with normal vision and a color blind male. Give the phenotypes and genotypes of the offspring including the sex of the children. Can any of them be color-blind?

10) Use a Punnett square to show the offspring of a cross between woman who carries the hemophilia gene and a man who is a hemophiliac. Give the phenotypes and genotypes of the offspring. Can any of their daughters be a hemophiliac?