Genetics•Mendel’s experiments and laws•Intermediate inheritance•Multiple alleles: the AB0 blood groups•Sex inheritance. Sex-linked inheritance

Genetics studies biological inheritance, the transmission of characters.

1. Introduction

•The first important results in the study of biological inheritance were obtained by Mendel (“Father of Genetics”) in 1865.•He worked with pea plants (Pisum sativum)

Keypoints of Mendel’s success:•He chose 7 distinct characters, and each character had 2 variants or traits clearly different.•He carried out experiments: he crossed specific individuals and observed the descendence.•He performed a quantitative treatment: elemental statistics.

Gregor Mendel (1822-1884)

The seven characters studied by Mendel in pea plants

1. Introduction

Mendel’s first experiment

Mendel selected true-breeding plants:a plant is said to be true-breeding for a trait of a character when the plant and all its descendants by self-fertilization show the same trait.

P.

Offspring

True-breeding plants

Self-fertilization

Self-fertilization

Round seed

Round seed

Round seed

Mendel’s first experiment

Mendel’s first law (uniformity of F1)The offspring which result of crossing two different true-breeding individuals (for a given character) are all similar (for that character).

Round seed Wrinkled seed P. ×

Round seed (all) F1.

(True-breeding plants)

Mendel’s first experiment: explanation•There are material factors (genes) which determine characters.•Genes come in pairs in individuals, but not in gametes.•Alternative forms of a gene are called alleles. Each allele determines a trait of a character.

Character: seed shape

Round seed: L

Wrinkled seed: l

Mendel’s first experiment: explanation

Character: seed shape

Round seed: L

Wrinkled seed: l

Round seed Wrinkled seed P. ×

Round seed (all) F1.

(True-breeding plants) LL ll

Gam. L l

Ll ????

Mendel’s first experiment: explanation•There are material factors (genes) which determine characters.•Genes come in pairs in individuals, but not in gametes.•Alternative forms of a gene are called alleles. Each allele determines a trait of a character.•An allele (A) is dominant over another (a) when, being together, only A is reflected in the phenotype, while a remains hidden.

Character: seed shapeRound seed: L

Wrinkled seed: l L is dominant over l

Mendel’s first experiment: explanation

Character: seed shapeRound seed: L

Wrinkled seed: l L is dominant over l

Round seed Wrinkled seed P. ×

Round seed (all) F1.

(True-breeding plants) LL ll

Gam. L l

Ll

Mendel’s second experiment

Round seed Wrinkled seed P. × Round seed (all) F1.

(True-breeding plants)

Round seed

Round seed (3/4) F2.

First experiment

F1.

Wrinkled seed (1/4)

Self-pollination (self-fertilization)Second

experiment

Mendel’s second experiment: explanation

Character: seed shape

Round seed: L

Wrinkled seed: l L is dominant over l

Round seed

Round seed (3/4) F2.

F1.

Wrinkled seed (1/4)

Ll

Gam.

♂ ♀ L L l l 1/2 1/2 1/2 1/2

Ll Ll LL ll 1/4 1/4

1/2

Mendel’s second law: segregationThe alleles, which were together in the plants, segregate (separate)

when gametes form, and combine randomly when fertilization takes place to produce the offspring.

Round seed

Round seed (3/4) F2.

F1.

Wrinkled seed (1/4)

Ll

Gam.

♂ ♀ L L l l 1/2 1/2 1/2 1/2

Ll Ll LL ll 1/4 1/4

1/2

The alleles segregate when gametes form

They combine randomly in the offspring

Mendel’s third experiment

Round, yellow seeds

F2.

F1.

Self-pollination (self-fertilization)

Round, yellow seeds

Wrinkled, green seeds

P. × (True-breeding plants)

Round, green seeds (3/16)

Round, yellow seeds (9/16)

Wrinkled, yellow seeds (3/16)

Wrinkled, green seeds (1/16)

Mendel’s third experiment: explanation

Round, yellow s. Wrinkled, green s.P. (true-breeding plants) ×

1/4

Gametes.

LLAA

♂ ♀

1/4 1/4 1/4 1/4 1/4 1/4 1/4

llaa

la LA

F1. LlAa Round, yellow s. (all)

LA La lA la LA La lA la Gametes.

Character: seed shape

Round seed: L

Wrinkled seed: l

L is dominant over l

Two characters:

Character: seed colour

Yellow seed: A

Green seed: a

A is dominant over a

LLAA (1) LLAa (1) LlAA (1) LlAa (1)

LLAa (1) LLaa (2) LlAa (1) Llaa (2)

LlAA (1) LlAa (1) llAA (3) llAa (3)

LlAa (1) Llaa (2) llAa (3) llaa (4)

LA La lA la

LA

La

lA

la

F2.

Round, yellow s. (L_A_) (1): 9/16Round, green s. (L_aa) (2): 3/16Wrinkled, yellow s. (llA_) (3): 3/16Wrinkled, green s. (llaa) (4): 1/16

Mendel’s third experiment: explanation

Mendel’s third law (independent assortment): Different characters are assorted independently. Thus, the traits of both characters are transmitted independently (not together, as in a block).

IMPORTANT: This law is only valid in some cases, not always.

Intermediate inheritance (=incomplete dominance) and codominance

In numerous characters there is no dominance between alleles. For example, flower colour in four-o’clock plants (Mirabilis jalapa).

Red flowers White flowers P. ×

Pink flowers (all) F1.

(True-breeding plants)

F2. White flowers (1/4)

Self-pollination (self-fertilization)

Red flowers (1/4)

Pink flowers (1/2)

Intermediate inheritance

Red flowers White flowers P. ×

Pink flowers (all) F1.

(True-breeding plants)

White flowers (1/4)

Red flowers (1/4)

Pink flowers (1/2)

Character: flower colour White

There is no dominance between CR and Cr 2 alleles: CR, Cr Red

Pink CRCR: red; CRCr: pink; CrCr: white

Gam. CR CW

F2.

Gam.

♂ ♀ CR CR CW CW 1/2 1/2 1/2 1/2

CRCR 1/4 1/4

1/2 CRCW CRCW CWCW

CRCW

CRCR CWCW

Multiple alleles: the AB0 blood groups

A gene may have more than two alleles in a population, although each individual can carry at most two alleles.

This is the case of the AB0 blood groups. There are 3 alleles which determine the different blood groups: IA, IB, i. Both IA and IB are dominant over i, but are codominant between them.Write the correspondence between genotypes and phenotypes:IAIA.-IAi.-IBIB.-IBi.-IAIB.-ii.-Problem. A man and a woman have four children. Each child belongs to a different blood group (AB0). What are the parents’ genotypes? Explain the cross, writing a complete diagram.

Sex inheritance. Sex-linked inheritance•In many organisms, sex is chromosomally determined.

-Autosomes: chromosomes not directly involved in sex determination.-Sex chromosomes: chromosomes involved in sex determination.

In human cells there are 22 pairs of autosomes and 1 pair of sex chromosomes.

Character: sexFemale: XX

Male: XY

Female (♀) Male (♂) P. ×

1/2 ♀F1.

XX XY

Gam. X X

XX

Y 1/2 1/2

XX 1/2 ♂

Sex inheritance. Sex-linked inheritanceHow is sex inherited?

In mammals and other animals...

Sex-linked inheritance

Two basic ideas:•Sex chromosomes contain genes. Many of these genes determine nonsexual characters, but these characters are sex-linked, their transmission is related to sex.•X and Y chromosomes are not totally homologous, but partially homologous. Each one contains one homologous segment and one differential segment. Then, in the differential segment of the X chromosome there are genes not represented in Y; in the differential segment of the Y chromosome there are genes not represented in X.

Sex-linked inheritance•X and Y chromosomes are not totally homologous, but partially homologous. Each one contains one homologous segment and one differential segment. Then, in the differential segment of the X chromosome there are genes not represented in Y; in the differential segment of the Y chromosome there are genes not represented in X.

Sex-linked inheritance

Hemophilia: A hereditary condition in which the blood does not clot properly.The gene which determines hemophilia (or normal clotting) is located in the differential segment of X chromosome (X-linked).h.- Allele which determines hemophilia (recessive).H.- Allele which determines normal clotting (dominant).XH.- X chromosome carrying the allele H.Xh.- X chromosome carrying the allele h.Y.- The Y chromosome cannot carry either H or h.

What are the possible genotypes?

Hemophilia

Sex-linked inheritance

What are the possible genotypes?XHXH.- “Normal” (as for blood clotting) woman.XHXh.- “Normal” woman, but she is a carrier (she carries the allele responsible for hemophilia).XhXh.-It does not exist. There are not hemophilic woman (but exceptions. The allele responsible for hemophilia (h) is lethal in homozygotes.XHY.- “Normal” (as for blood clotting) man.XhY.- Hemophilic man.

Hemophilia

Sex-linked inheritance

Colour-blindness: A hereditary condition in which some colours are mistaken.The gene which determines colour-blindness (or normal colour perception) is located in the differential segment of X chromosome (X-linked).d.- Allele which determines colour-blindness (recessive).D.- Allele which determines normal vision (dominant).XD.- X chromosome carrying the allele D.Xd.- X chromosome carrying the allele d.Y.- The Y chromosome cannot carry either D or d.

What are the possible genotypes? Write them. Take into account that d allele is not lethal in homozygotes, so there are colour-blind women.

Colour-blindness (daltonism)

Transmission of hemophilia: Genealogical tree of Queen Victoria (England)

Sex-linked inheritance