Mendel genetic laws

MENDIAL’S LAW OF GENETICS Opening activity: How do your parents look like? ¿Why are you like your mother or father? Why you are not?

Activity

objectives

Opening activity

Development How

do

Time your

15 min

parents look like? ¿Why are you like your

mother

father?

Why

or you

are not?

Introduction

to

Mendelian genetics

Explanation of the 10 min first steps Mendel did

First law of Mendel

Explanation of the 20 min first

law

and

exercises

1. Introduction to Mendelian genetics Gregor Mendel was an Austrian monk and gardener who carried out a significant study of heredity. Mendel used pea plants to discover the mechanism of heredity – how traits get passed from parents to offspring.

This experiment drove him to

discover several important things about heredity, that is why he is known as the Father of genetics.

Mendel used peas because t hey reproduce sexually, they have two distinct, male and female, sex cells called gametes and their traits were easy to isolate After 1000s of experiments, Mendel came up with some “rules of heredity”:

Genotype and phenotype •

Phenotype - the way an organism looks •

•

Example - red hair or brown hair

genotype - the gene combination of an organism •

AA or Aa or aa

Heterozygous and Homozygous •

Heterozygous - if the two alleles for a trait are different (Aa)

•

Homozygous - if the two alleles for a trait are the same (AA or aa)

Dihybrid and Monohybrid •

Dihybrid Cross - crossing parents who differ in two traits (AAEE with aaee)

•

Monohybrid Cross - crossing parents who differ in only one trait (AA with aa)

2. 1 First law of Mendel This law is also called the Law of uniformity of hybrids of the first generation (F1), and asserts that when you cross two varieties of pure breed individuals, both homozygotes, for a certain character, all hybrids of the first generation are equal.

The individuals of this first generation filial (F1) are heterozygous or hybrid, because their genes alleles carry information of the two pure or homozygous races: the dominant, which manifests, and recessive, which does not.

Mendel came to this conclusion working with a pure variety of pea plants that produced the yellow seeds and with a variety that produced the green ones. After crossing these plants, he always obtained plants with yellow seeds.

2.1. Activity If a homozygous tall stem (AA) plant is crossed with a homozygous dwarf stem (aa), Knowing that the tall stem is dominant on the dwarf stem, what will be the genotypes and phenotypes of F1 and F2?

When crossing two black flies you get an offspring formed by 216 black flies and 72 white Representing by NN the color black and by nn the color white, reason the crossing and what will be the genotype of the flies that cross and of the offspring obtained.

Curly hair in dogs dominates over straight hair. A pair of curly hair had a puppy hair also curly and you want to know if it is heterozygous. With what kind of females will to cross? Reason said crossing.

Activity Opening activity

objectives



Development

Make clear their Review of the 1st

Time 15 min

nd ideas about 1st and and 2 law

2nd law 3rd law of Mendel



Understanding

Explanation of the 20 min

the principles of 3rd law of Mendel the 3rd Mendelian law Exercises of the



3rd law

Be sure they can Exercises

20 min

put in to practise their

knowledge

about mendelian laws Homework

Create

Encourage students

a

about genealogical tree

their heredity

from your great grand parents.

Color of the eyes and hair.

3.1 2nd Law of Mendel.

The Law of Segregation

Goes like so: During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

The way I figure it, Mendel probably got really bored crossing pure dominant trait pea plants with pure recessive trait pea plants (over & over & over again) & getting nothing but pea plants with the dominant trait as a result. Except for gaining more & more evidence for his Law of Dominance, this probably grew tiresome. So, at one point he takes the offspring of a previous cross & crosses them.

Recall that his original cross for the tall & short pea plants was:

So, he takes two of the "F1" generation (which are tall) & crosses them. I would think that he is figuring that he's gonna get all tall again (since tall is dominant).

But no! Low & behold he gets some short plants from this cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall & 1/4 short. So he says to himself, "Greg ol' boy, the parent plants for this cross each have one tall factor that dominates the short factor & causes them to grow tall. To get short plants from these parents, the tall & short factors must separate, otherwise a plant with just short factors couldn't be produced. The factors must SEGREGATE themselves somewhere between the production of sex cells & fertilization."

I think it's easier to picture this law by using a p-square. Our cross is two hybrid parents, Tt x Tt. The punnet square would look like this:

Now, when completing a Punnet Square, we model this "Law of Segregation" every time. When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait. In real life this happens during a process of cell division called "MEIOSIS".

Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm. Sometimes the term "GAMETOGENESIS" is used instead of meiosis.

Scientists love vocabulary (sorry).

You can see from the p-square that any time you cross two hybrids, 3 of the 4 boxes will produce an organism with the dominant trait (in this example "TT", "Tt", & "Tt"), and 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype ("tt" in this example).

Summary:

3.2 Activities.

4.1 3rd law of Mendel Mendel's third law, the law of independent assortment, states that the way an allele pair gets segregated into two daughter cells during the second division of meiosis has no effect on how any other allele pair gets segregated. In other words, the traits inherited through one gene will be inherited independently of the traits inherited through another gene because the genes reside on different chromosomes that are independently assorted into daughter cells during meiosis.

To know the influence that two or more characters can have on each other when they are inherited, he devised the following experiment.

He chose pure races for two different characters, which had already studied how they were inherited, but separately. Now I would study them together. First he made the following crossing: P: Yellow and smooth seed x Green and rough seed

F1: 100% Yellow and smooth seed.

Then he verified that for two characters his first law was also valid, since all the descendants were equal to each other, and equal to one of his parents.

Then, he crossed two dihybrid individuals of the F1 or self-fertilized, and obtained a second filial generation (F2) composed of 556 seeds with the following characteristics: 315 yellow and smooth seeds.

108 yellow and rough seeds.

101 green and smooth seeds.

32 green and rough seeds.

These results correspond to approximate relative proportions of 9: 3: 3: 1. That is, of 16 individuals, 9 would be obtained with dominant dihomozygotes, with the two dominant characteristics, 3 with a dominant character and another recessive, another 3 with the other dominant character and the other recessive, and a dihomozygous, doubly recessive for both characters.

Thus, Mendel proposed his third law or the independent inheritance of nonantagonistic characters, which states that the different characters are inherited independently of each other.

With these results he observed something that did not happen when he studied a single character: Individuals other than the progenitors appeared, such as pea plants with green and rough seeds and yellow and smooth ones.

Proportions were obtained 9: 3: 3: 1.

Mendel came to the conclusion that just as hereditary factors are independent, so are the characters, so they can be combined in all possible ways, appearing combinations that did not exist before.

We have to take into account that Mendel's findings occurred without him having the knowledge we have of DNA.

4.2. Activity In a certain plant species, the red flowers are sewed to a dominant gene with respect to the allele that produces white flowers, and the flared shape of them is dominant with respect to the tubular shape of the flowers. Knowing that both pairs of alleles are transmitted independently, it indicates: a) The percentage of diheterozygous plants that can be expected from crossing plants with red (heterozygous) and tubular flowers with diherterozygous plants. b) The probability of obtaining a double homozygous recessive plant