This document provides an overview of biological inheritance. It discusses genes and alleles, Mendel's laws of inheritance from his early genetic studies with pea plants, the location of genes on chromosomes, determining sex based on sex chromosomes, inheritance linked to sex chromosomes, and mutations. The key topics covered are Mendel's principles of uniformity, segregation, and independent assortment which established the foundations of modern genetics. It also examines gene location and linkage, sex determination, sex-linked inheritance patterns, and the types and causes of genetic mutations.

1. UNIT 5: BIOLOGICAL
INHERITANCE

2. INDEX
1. Genes
2. Early GeneticStudies
3. Special cases
4. Location of Genes
5. DeterminingSex

3. 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)

4. 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)

5. Example 1: For the trait “hair colour in guinea pigs”: Two alleles are possible:
N = black  dominant allele
n = 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

6. 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 are
co-dominant alleles and cause
INTERMEDIATE INHERITANCE

7. 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
Seeds
Smooth Wrikled
Yellow Green
Flowers Red White
Stems Long Short
He started working with purebred individuals (homozygous individuals)
(NN or nn)

8. 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

9. Principle of Segregation
Gametes produced
Gametes produced
When two heterozygous individuals from the F1 are crossed, in the F2 appear again the two
phenotypes of the Parentals
Genotypes probabilities:
 AA 25%
 Aa 50%
 aa 25%
Phenotypes probabilities:
 Yellow 75%
 Green 25%

10. 2nd Mendel’s law: Principle of Independant assortment
When two individuals that have two or more different traits are crossed, the
transmission of each trait is independent from the other traits
This only works for non-linked traits!!
A = yellow
a = green
A>a
B = smooth
b = wrinkled
B>b
Possible gametes: AB ab
Possible gametes: AB Ab aB ab

11. Phenotypes probabilities:
Yellow smooth (A_B_)
Yellow wrinkled (A_bb)
Green smooth (aaB_)
Green wrinkled (aabb)
9/16
3/16
3/16
1/16
9:3:3:1
Page 91 activities 3, 4, 8, 9, 10 and 11

12. 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 two
homologous chromosomes.
Drosophila melanogaster
(1 locus/ many loci)

13. Some genes and their locus on chromosome 7

14. 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

15. 5. Determining Sex
In many animals there are different chromosomes in males and females:
In human
being
2 sex
chromosomes
XY/XX
44 autosomes
Common 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, 23
Page 91 activities 5 and 7

16. Human female karyotype
Human male karyotype
Karyotype = the chromosomes of a cell,
usually displayed as a systematized arrangement
of chromosome pairs in descending order of size.

17. 6. Inheritance Linked to Sex
A 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?

18. 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%

19. 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

20. Numerical mutations
Karyotype from a male with Klinefelter syndrome

21. 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

22. 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