16.2.3
Linkage
Autosomal Linkage
Autosomal Linkage
Linkage is when genes that are close to one another on a chromosome are likely to be inherited together. Linkage shows that some allele combinations are not inherited independently of each other.


Mendel's law
Mendel's law
- Mendel's law of independent assortment states that genes do not influence the sorting of alleles into gametes. This is not always the case.
- Some allele combinations are not inherited independently of each other.
- Genes that are located close to each other on the same chromosome are more likely to be inherited as a pair.
- This is called linkage.


Autosomes
Autosomes
- Autosomes are all the chromosomes except the sex chromosomes (X and Y).
- Autosomes are arranged in pairs called homologous chromosomes (one from the father and one from the mother).
- Homologous chromosomes consist of the same genes in the same order along the chromosome.
- There is some variation if the chromosomes have different alleles but the genes are the same.


Recombination
Recombination
- When gametes are produced by meiosis, multiple sections of homologous chromosomes are exchanged in a process called recombination.
- If two genes are located in close proximity on the same chromosome, they are more likely to be exchanged together and not separated in recombination.
- The genes are more likely to be transmitted to a gamete together.


Linkage
Linkage
- Genes that are close together on the same autosome are more likely to be transmitted to a gamete together.
- The genes are linked. This is autosomal linkage.
Sex Linkage
Sex Linkage
Linkage is where genes that are close to one another on a chromosome are likely to be inherited together. Sex linkage is different from autosomal linkage because it takes place in the sex chromosomes.


Sex chromosomes
Sex chromosomes
- In many organisms, the sex chromosomes (X and Y) determine the sex of an individual.
- Sex chromosomes differ from autosomes because they are non-homologous.
- Non-homologous chromosomes do not consist of the same genes in the same order along the chromosome.


Y chromosome
Y chromosome
- Human females have a homologous pair of X chromosomes (XX) and human males are heterozygous (XY).
- X and Y chromosomes contain a small region of similarity.
- The Y chromosome is considerably shorter than the X chromosome and contains fewer genes.
- If a gene is only found on the X chromosome, it is X-linked.


X-linked genes
X-linked genes
- Males only have one copy of an allele for X-linked genes.
- There is no allele for the same gene on the Y chromosome. This is called hemizygosity.
- Hemizygosity means that there is no role of dominance and recessiveness.
- If a recessive gene is inherited on the X chromosome, it will always be present in the phenotype as it is the only allele present.


Haemophilia
Haemophilia
- Haemophilia is a blood disorder that is X-linked.
- Humans males only need to inherit one recessive mutant X allele to be affected by the disorder but females must inherit two copies of the recessive allele.
- This means haemophilia is more common in males.
- Females can be carriers for the disorder when they are heterozygous.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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