UNIT 5: BIOLOGICAL INHERITANCE

INDEX1. Genes2. Early Genetic Studies3. Special cases4. Location of Genes5. Determining Sex

1. Genes

Hereditary trait

• characteristic which can be passed on to the offspring.

Gene

• Unit that transmits genetic information. It is made up of DNA.

Genotipe

• set of genes that an individual has

Phenotype

• Set of observable traits that an individual has (it can be influenced by the environment)

(caracteres hereditarios/características hereditarias)

A gene can have different variants, called alleles.

An individual has 2 alleles for each trait: one from the father and one from the mother

If both alleles are the same for

a trait (purebred)

Homozygous individual

If both alleles are different for this trait (hybrid)

Heterozygous individual

(individuo homocigoto/heterocigoto)

(Línea pura)

Example 1: For the trait “hair colour in guinea pigs”: Two alleles are possible:N = black dominant allelen = white recessive allele

GENOTYPE PHENOTYPE

NN dominant homozygous Black

nn recessive homozygous White

Nn heterozygous black

The dominant allele prevent the recessive allele from appearing

Example 2: For the trait “flower colour in morning glory flower” two alleles are possible: N1 (red) and N2 (white)

GENOTYPE PHENOTYPE

N1N1 (homozygous) Red

N2N2 (homozygous) White

N1N2 heterozygous Pink!!

N1N1 N2N2

N1N2

N1 and N2 are equally strong, they areco-dominant alleles and cause

INTERMEDIATE INHERITANCE

2. Early Genetic Studies

Gregor Mendel was an Austrian monk (2nd half of the 19th century)

His discoveries form the basis of modern genetics.He worked with pea plants (easy to see traits and to handle)He did lots of crossings between pea plants with different traits

probabilities

SeedsSmooth Wrikled

Yellow Green

Flowers Red White

Stems Long Short

He started working with purebred individuals (homozygous individuals)

(NN or nn)

1st Mendel’s Law: Principle of Uniformity

When two purebred individuals are crossed, all of their offspring (F1) will have identical phenotypes.

They will also have identical genotypes: heterozygous

P: parents

Gametes produced

F1: first filial generation

Page 88 activities 1 and 2

Yellow pea Green pea

Yellow peas

Principle of Segregation

Gametes produced

Gametes produced

When two heterozygous individuals from the F1 are crossed, in the F2 appear again the twophenotypes of the Parentals

Genotypes probabilities:

AA 25% Aa 50% aa 25%

Phenotypes probabilities:

Yellow 75% Green 25%

2nd Mendel’s law: Principle of Independant assortment

When two individuals that have two or more different traits are crossed, thetransmission of each trait is independent from the other traits

This only works for non-linked traits!!

A = yellowa = greenA>a

B = smoothb = wrinkledB>b

Possible gametes: AB ab

Possible gametes: AB Ab aB ab

Phenotypes probabilities:Yellow smooth (A_B_)Yellow wrinkled (A_bb)Green smooth (aaB_)Green wrinkled (aabb)

9/163/163/161/16

9:3:3:1

Page 91 activities 3, 4, 8, 9, 10 and 11

4. Location of Genes

Morgan (1915) developed a theory about the role of chromosomes in inheritance:

Chromosomal theory of heredity

(teoría cromosómica de la herencia)

• Genes are located on chromosomes on a specific place (locus).

• Each gene is made up of a segment of DNA.

• 2 alleles which determine a specific trait are located on twohomologous chromosomes.

Drosophila melanogaster

(1 locus/ many loci)

Some genes and their locus on chromosome 7

All the genes located on the same chromosome are linked to each other and are transmitted together,

specially if they are not very far away from each other

Page 95 activities 15, 16, 17, 23

5. Determining Sex

In many animals there are different chromosomes in males and females:

In human being

2 sex chromosomes

XY/XX

44 autosomesCommon to both sexes

Genes not related to sexual differences

Genes that take part in the development of the male

XX XY Page 95 activities 15, 16, 17, 23Page 91 activities 5 and 7

Human female karyotype

Human male karyotype

Karyotype = the chromosomes of a cell,usually displayed as a systematized arrangementof chromosome pairs in descending order of size.

6. Inheritance Linked to SexA trait linked to the X chromosome is a trait controlled by a gene located on the X chromosome.

Examples: daltonism (colour blindness) and haemophilia

Individuals Female Male

XX (healthy) XY (healthy)

XdXd (ill) XdY (ill)

XdX (carrier but healthy)

Problem: A daltonic man wants to have babies with a healthy woman. What are the probabilities to get a daltonic boy?

Man with daltonism x healthy woman

Xd Y

X XdX XY

Genotypes: XdY XX

Problem 2: A healthy man wants to have babies with a carrier woman. What are the probabilities to get a daltonic boy?

Sol.: all boys are healthy and all girls are carrier The probability to obtain a daltonic boy is 0%.

Genotypes: XY XdX

X Y

X XX XY

Xd XdX XdY

50% descendants are daltonic(50% of men)Sol.: The probability to obtain a daltonic boy is 25%

7. Mutations

Mutations are unexpected and random changes in genes or DNA sequence.

Mutation types

Gene mutation

The alteration affects the chemical

structure of the DNA.

Chromosome mutation

The alteration affects the

structure of the chromosomes.

Numerical mutation

The alteration affects the number of

chromosomes.

Chromosome mutation

Gene mutation

Numerical mutations

Karyotype from a male with Klinefelter syndrome

Causes of mutations

Physical mutagens

Radiation (X-rays, UV rays from the

Sun…)

Chemical mutagens

Pollutants, tobacco smoke, some pestices…

Biological mutagens

Some viruses like Human Papilloma

Virus

Page 101 activities 26, 27, 31, 32

Page 102 activities 2, 8 and 10

Development of Basic Competences page 103 (ALL)

Glossary:

• Purebred

• Intermediate inheritance

• Genotype

• Phenotype

• Allele

• Trait