Unit 4:  The genetic revolution                                                       1   2   3   4  

1. GENETICS

1.1. Mendelian genetics

Gregor Johann Mendel was born in 1822 in Heinzendorf (Austria) and entered in the Augustinians order in 1843. He was very interested in the way genetic traits were transmitted between generations. He worked during long time in the orchard of his convent in Brno (Czech Republic) crossing different varieties of peas (Pisum sativum).

Over years, he collected a huge amount of data about the frequencies with which each studied trait of the plant was transmitted (he studied seven different features). From these data, he elaborated a set of laws about the transmission of the biological characteristics, today known as Mendel’s Laws.

In spite of the importance of his work, it was ignored. Mendel published his discoveries in a short diffusion magazine and in addition, in this moment (1866) the scientific interest was very focused on the controversy created around the evolutionary theories of Lamarck and Darwin.

The unchanging quality of the hereditary factors of Mendel did not seem compatible with the evolution of the species and what really was the scientific explanation of this biological process of change was disregarded.

More than thirty years later in one of the most surprising coincidences of the scientific investigation, three scientists, the Dutch De Vries, the German Correns and the Austrian Tschennak, rediscovered separately in the same year (1900) the Mendel’s Laws.

a) Concepts of Mendelian Genetics

A gene is the minimal unit of genetic information. Each one contains information for one inheritance trait. The inheritance traits are the physical or physiological characteristics of an individual, the eye’s colour, for instance.   

Genes can have alternative ways. Each one is called allele (e.g. the colour of the eye can be blue or brown)     

Most part of organisms are diploids, that is to say they have two set of chromosomes (two copies of each chromosome) The members of each couple of chromosomes are called homologous. The homologous chromosomes have the same genes, although they can have different alleles.

-     An individual is homozygous (or pure breeding) if both alleles are identical for a trait. 

-     An individual is heterozygous (or hybrid) if both alleles are different for a trait.

In heterozygous individuals, usually one allele is expressed while the other not. The expressed allele is called dominant (It is represented by capital letters, e.g. “A”) and the non-expressed one recessive (It is represented by lower-case letters, e.g. “a”).

This provokes that many alleles present in the individual never were expressed. The genotype is the set of alleles that an individual has while its phenotype is the set of traits that expresses.

b) Mendel's laws

Today we have reformulated the Mendel’s Laws including in them the current knowledge in Genetics.

• Mendel’s first law: the uniformity of the first filial generation (F₁).

In his first experiment Mendel crossed two homozygous individuals (pure breeding) for one trait (e.g. the colour of the seed) He called parental generation to parents and filial generation (F₁) to the offspring.

When a dominant homozygous individual of yellow seed (AA) is crossed with other recessive homozygous individual of green seeds (aa), all the individuals of the offspring are heterozygous of yellow seeds (Aa). The plant AA produces gametes that have the allele A. The gametes of the plant aa have the allele a. The zygote formed by the joint of both gametes will be Aa.

• Mendel’s second law: the segregation of traits in the second filial generation (F₂)

In his second group of experiments, Mendel crossed the hybrids (Aa) obtained in the F₁ generation (self-fertilisation). In this case each individual can produce two types of gametes, one type carries the allele A and the other, the allele a. After fertilisation the second filial generation (F₂) had combinations of alleles that did not exist in F₁. The proportion is one green seeded plant (recessive) to three yellow seeded plants (dominant) (3:1).

• Mendel’s third law: the independent assortment of traits.

For his third group of experiments, Mendel chose homozygous individuals which differed in two traits: colour and shape of seeds. Parental generation had the following genotypes: AABB (yellow smooth) y aabb (green wrinkled). Both progenitors can produce only one type of gamete, “AB” and “ab” respectively.

The fusion of these gametes generates a uniform F₁, formed by di-heterozygous individuals of AaBb genotype and yellow smooth phenotype.

Mendel crossed these individuals of F₁ among them. Di-heterozygous individuals can form four different types of gametes: AB, Ab, aB and ab. So, he obtained a F₂ formed by individuals that presented the traits of the parental generation combined in the proportion 9:3:3:1.

In the transmission of two or more traits, each one is transmitted in an independent way and they combine at random in the offspring.

1.1.  Indicate which genotypes could be possible for a trait in a trait determined by a gene with two different alleles (A and a).

1.2. Is it possible that two individuals have different phenotype and the same genotype?

            And different genotype and the same phenotype? Give reasons for your answers.

1.3.  Gametes are the specialised cell of sexual reproduction. They form during a special cellular division called meiosis. During this process the number of chromosomes is reduced at a half (one set). In this way when male and female gametes fuse to form a new individual, it will have the species’ typical number of chromosomes (two sets). How many different kinds of gametes can form an individual of genotype AA? And other of genotype Aa?

1.4. Indicate the expected genotypic and phenotypic proportion of a crossing between black mice and brown mice. Black is the dominant allele for the colour of hair and both individuals are homozygous. What is the probability to find brown individuals if we cross the descendants of the previous crossing among them?

1.5.  What is the probability to obtain di-heterozygous individuals from two progenitors which genotypes are AARR y aarr? Knowing that the allele A determines tall stem and the allele R, red colour, is there any possibility to find descendants of short stem and white colour in F₂?

1.6. What is the probability to obtain di-heterozygous individuals from two progenitors which genotypes are AARR y aarr? Knowing that the allele A determines tall stem and the allele R, red colour, is there any possibility to find descendants of short stem and white colour in F₂?