IGCSE BIOLOGY SECTION 3 LESSON 3

Content Section 3 Reproduction and Inheritance a)Reproduction - Flowering plants - Humans b) Inheritance

Content Lesson 3 b) Inheritance 3.13 understand that the nucleus of a cell contains chromosomes on which genes are located 3.14 understand that a gene is a section of a molecule of DNA and that a gene codes for a specific protein 3.15 describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G) 3.16 understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics 3.17 understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance 3.18 describe patterns of monohybrid inheritance using a genetic diagram 3.19 understand how to interpret family pedigrees 3.20 predict probabilities of outcomes from monohybrid crosses

The nucleus The nucleus of the cell, containing all of the genetic material. This material is inherited from the parents.

The nucleus The nucleus contains chromosomes – in normal human cells, there are 23 pairs of chromosomes. Each chromosome is made up of a very special molecule called DNA.

The nucleus The nucleus contains chromosomes – in normal human cells, there are 23 pairs of chromosomes. Each chromosome is made up of a very special molecule called DNA. DNA stands for deoxyribonucleic acid, but at this stage just stick with the initials DNA!

The nucleus Here is an individual chromosome ( the x- chromosome). Inside, the double helix of DNA can be clearly seen.

Chromosomes, genes and DNA chromosome

Chromosomes, genes and DNA Individual sections of a chromosome are called genes. Each gene ( a short section of DNA) codes for a particular protein, which may control particular characteristics, such as eye colour. Each chromosome may contain thousands of genes.

DNA Structure DNA consists of two strands, wrapped into a double helix.

DNA Structure DNA consists of two strands, wrapped into a double helix. The two strands are linked by pairs of BASES There are four bases – adenine, thymine, cytosine and guanine.

DNA Structure DNA consists of two strands, wrapped into a double helix. The two strands are linked by pairs of BASES There are four bases – adenine, thymine, cytosine and guanine. Adenine + Thymine Cytosine + Guanine

DNA Structure DNA consists of two strands, wrapped into a double helix. The two strands are linked by pairs of BASES There are four bases – adenine, thymine, cytosine and guanine. Adenine + Thymine Cytosine + Guanine A T C G

DNA Structure DNA consists of two strands, wrapped into a double helix. The two strands are linked by pairs of BASES There are four bases – adenine, thymine, cytosine and guanine. Adenine + Thymine Cytosine + Guanine A T C G DNA molecules form a complete set of instructions on how an organism should be ‘constructed’ and how the cells should work.

DNA Structure The bases are ‘read’ in threes, or triplets.

DNA Structure The bases are ‘read’ in threes, or triplets. Each triplet codes for a particular amino acid.

DNA Structure The bases are ‘read’ in threes, or triplets. Each triplet codes for a particular amino acid. Don’t forget that proteins are made up of amino acids!

DNA Structure The bases are ‘read’ in threes, or triplets. Each triplet codes for a particular amino acid. So this triplet of bases is cytosine – cytosine – thymine or CCT

DNA Structure The bases are ‘read’ in threes, or triplets. Each triplet codes for a particular amino acid. So this triplet of bases is cytosine – cytosine – thymine or CCT TATGGATGTGCTACCTCG

DNA Structure TATGGATGTGCTACCTCG Since there are only about 20 different amino acids that make up all the protein chains, the different base triplet combinations are more than sufficient

Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly.

Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly. Occasionally a mistake may occur, and bases may be put in the wrong order.

Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly. Occasionally a mistake may occur, and bases may be put in the wrong order. As a result, there will be a different sequence of amino acids, and therefore a different protein will be made.

Genetic mutations Every time a cell divides, all the DNA in the nucleus must be copied exactly. Occasionally a mistake may occur, and bases may be put in the wrong order. As a result, there will be a different sequence of amino acids, and therefore a different protein will be made. This change in the order of the bases is called a MUTATION

Genetic mutations CausesEffects Mutations occur naturally but …… there is an increased risk if individuals are exposed to mutagenic agents such as ionising radiation (UV, X-rays) radioactive substances and certain chemicals. the greater the dose, the greater the risk. Most mutations are harmful and in … Reproductive cells can cause death or abnormality in body cells they may cause cancer some mutations are neutral and some may increase the survival chances of an organism and its offspring who inherit the gene

Alleles Let’s just recap a second!

Alleles Let’s just recap a second!

Alleles Let’s just recap a second! Genes control specific characteristics, such as eye colour

Alleles How many different eye colours are there?

Alleles How many different eye colours are there? BluebrownGreyHazel

Alleles How many different eye colours are there? BluebrownGreyHazel There are different forms of the same gene, all coding for different eye colours.

Alleles How many different eye colours are there? BluebrownGreyHazel There are different forms of the same gene, all coding for different eye colours. These different forms of the same gene are called alleles.

Alleles How many different eye colours are there? BluebrownGreyHazel There are different forms of the same gene, all coding for different eye colours. These different forms of the same gene are called alleles. So there are alleles for blue eyes, brown eyes, etc.

Remember that we inherit specific genes from both parents.

So we will inherit one eye colour gene from our mother, and another eye colour gene from our father (remember that different forms of the same gene are called alleles)

It’s definition time!

DOMINANT -when a pair of alleles (or genes) are present, each coding for a particular characteristic, the dominant allele is the one that shows. For example, the brown eye colour allele is dominant over the blue allele, so an individual with both blue and brown alleles will have brown eyes.

RECESSIVE - the recessive allele will only have an effect when the dominant allele is missing. For example, if you inherit the blue allele from your mother and the blur allele from your father, then you would have blue eyes (there is no other allele present to ‘dominate’ the blue allele).

Dominant alleles are shown using capital letters. For example, the brown eye allele is ‘B’

Dominant alleles are shown using capital letters. For example, the brown eye allele is ‘B’ Recessive alleles are lower case. For example, the blue eye allele is ‘b’

+ Sperm Egg Zygote

+ Sperm Egg Zygote Contains half the chromosome number of normal body cells Eg. 23 in humans Contains half the chromosome number of normal body cells Eg. 23 in humans Contains the full chromosome number Eg. 46 in humans

+ Sperm Egg Zygote B b Bb

Homozygous. If both chromosomes in a pair contain the same allele of a gene then the individual is homozygous for that gene or condition. eg. BB or bb

Heterozygous. If the chromosomes in a pair contain different alleles of a gene then the individual is said to be heterozygous for that gene or condition. eg. Bb (bB)

Inheritance terminology Homozygous dominant Heterozygous Homozygous recessive Tongue rolling TT (can roll)Tt (can roll)tt (can’t roll) Eye colour BB (brown)Bb (brown)bb (blue) Ear lobes EE (free lobes) Ee (free lobes) ee (attached lobes)

Phenotype The phenotype describes the outward appearance of an individual. eg. BB or Bb individuals will both have brown eyes.

Genotype The genotype describes the actual genes present in an individual eg. BB, Bb or bb

Co-dominance This refers to a situation when both alleles are clearly visible and do not overpower each other in the phenotype. eg. the ‘A’ and ‘B’ alleles are co-dominant in producing the ‘AB’ blood group phenotype.

OK, let’s move on now and consider monohybrid inheritance

Monohybrid what …???

When a characteristic is determined by a single pair of alleles, then a simple genetic diagram can be shown. This type of inheritance is referred to as monohybrid inheritance.

As an example, let’s look at a genetic cross for two parents – one is homozygous for brown eyes (BB) and the other is homozygous for blue eyes (bb)

BBbb Brown eyesBlue eyes Parents x x Gametes BBbb Offspring Bb Brown All four offspring are heterozygous (Bb) for brown eyes

Bb B b Punnett Square Gametes Heterozygous brown eyed mother Heterozygous brown eyed father

Bb BBBBb b bb 3 brown eyed offspring and one blue eyed offspring. 3:1 ratio Gametes Heterozygous brown eyed mother Heterozygous brown eyed father

bb BBb bbb 2 brown eyed offspring and two blue eyed offspring. 1:1 ratio Gametes Homozygous blue eyed mother Heterozygous brown eyed father

Golden rules of monohybrid crosses Monohybrid crossOutcome Parent 1: homozygous dominant (eg. TT) Parent 2: heterozygous recessive (eg. tt) All offspring will be heterozygous and show the dominant characteristic. (eg. Tt)

Golden rules of monohybrid crosses Monohybrid crossOutcome Parent 1: homozygous dominant (eg. TT) Parent 2: heterozygous recessive (eg. tt) All offspring will be heterozygous and show the dominant characteristic. (eg. Tt) Parent 1: heterozygous dominant (eg. Tt) Parent 2: homozygous recessive (eg. tt) 50% of offspring will be heterozygous dominant (Tt) and 50% will be homozygous recessive (tt) Ratio 1:1

Golden rules of monohybrid crosses Monohybrid crossOutcome Parent 1: homozygous dominant (eg. TT) Parent 2: heterozygous recessive (eg. tt) All offspring will be heterozygous and show the dominant characteristic. (eg. Tt) Parent 1: heterozygous dominant (eg. Tt) Parent 2: homozygous recessive (eg. tt) 50% of offspring will be heterozygous dominant (Tt) and 50% will be homozygous recessive (tt) Ratio 1:1 Parent 1: heterozygous dominant (eg. Tt) Parent 2: heterozygous dominant (eg. Tt) 25% of offspring will be homozygous dominant (TT), 50% will be heterozygous dominant (Tt), and 25% will be homozygous recessive (tt). Phenotype ratio 3:1

Content Lesson 3 b) Inheritance 3.13 understand that the nucleus of a cell contains chromosomes on which genes are located 3.14 understand that a gene is a section of a molecule of DNA and that a gene codes for a specific protein 3.15 describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G) 3.16 understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics 3.17 understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance 3.18 describe patterns of monohybrid inheritance using a genetic diagram 3.19 understand how to interpret family pedigrees 3.20 predict probabilities of outcomes from monohybrid crosses

Understand how to interpret family pedigrees.

Pedigree charts are normally used to show disease down a family tree. For example, the inheritance of sickle cell anaemia.

Doctors can use a pedigree analysis chart to show how genetic disorders are inherited in a family.

They can use this analysis to work out the probability (chance) that someone in the family will inherit the condition.

Pedigree analysis Male with sickle cell disease Normal male Female with sickle cell disease Normal female

Pedigree analysis

Both parents are sufferers

Pedigree analysis Both parents are sufferers Of their 4 children, 3 have sickle cell disease

Pedigree analysis Both parents are sufferers Of their 4 children, 3 have sickle cell disease If one of the affected offspring marries a normal male, what are the chances that their children will inherit the disease?

End of Section 3 Lesson 3 In this lesson we have covered: Chromosomes, genes and DNA. Genetic mutations. Alleles. Genetic inheritance. Monohybrid crosses.