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UNIT 5: BIOLOGICAL INHERITANCE
INDEX1.Genes2.Early Genetic Studies3.Special cases4.Location of Genes5.Determining Sex
1. GenesHereditary 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 PHENOTYPENN 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 PHENOTYPEN1N1 (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 StudiesGregor 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)
2.1 Mendel’s Laws
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 two phenotypes 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, the transmission 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. Chromosomal theory of heredity
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)
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 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, 23Page 91 activities 5 and 7
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.
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 for haemophilia. What are the probabilities to get a haemophilic 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 XhX XhY
(50% of men will be haemophile)Sol.: The probability to obtain a haemophilic boy is 25%.
Family tree: Haemophilia in the Borbons
http://biogeogc.wikispaces.com/Gen%C3%A9tica
7. MutationsMutations 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