Download
1 / 72
960 likes | 1.32k Views
iGCSE Biology Section 3 lesson 3. Content. Section 3 Reproduction and Inheritance. Reproduction - Flowering plants - Humans b) Inheritance. Content. Lesson 3 b) Inheritance. b) Inheritance.
E N D
Content Section 3 Reproduction and Inheritance • Reproduction • - Flowering plants • - Humans • b) Inheritance
Content Lesson 3 b) Inheritance 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. A T C G 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 DNA molecules form a complete set of instructions on how an organism should be ‘constructed’ and how the cells should work. There are four bases – adenine, thymine, cytosine and guanine. A T C G Adenine + Thymine Cytosine + Guanine
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
DNA Structure 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
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?
Alleles There are different forms of the same gene, all coding for different eye colours. How many different eye colours are there?
Alleles There are different forms of the same gene, all coding for different eye colours. How many different eye colours are there? These different forms of the same gene are called alleles.
Alleles There are different forms of the same gene, all coding for different eye colours. How many different eye colours are there? These different forms of the same gene are called alleles. So there are alleles for blue eyes, brown eyes, etc.
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)
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 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 Zygote Contains the full chromosome number Eg. 46 in humans
+ Sperm Egg b B Zygote 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)
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.
