What is an organism’s ‘Genotype’
The genetic constitution of an organism
- Includes all the alleles that an organism has
- Determines the limits, within which an individual may vary
eg. Genes mean max height is 6ft, but actual height is effected by other factors, such as diet
What is an organism’s ‘Phenotype’
The observable/biochemical characteristics of an organism
- Result of the interaction between the expression of the genotype and the environment
- ∴ Altered by environmental factors
eg. Maximum potential height may not be reached, due to factors such as a calcium deficiency
What is a ‘Gene’
Length of DNA (sequence of nucleotides) that codes for a protein
- Codes for polypeptide that is folded into protein
- Produced proteins determine the phenotype
What is a ‘locus’
The position of a gene on a chromosome
What is an ‘Allele’
One of the several different forms of a gene
- Only one allele of a gene can occur at the locus of a chromosome
- Alleles can be recessive or dominant
- In diploid organisms with homologous chromosomes (2 copies of each), there are 2 loci for each gene, each with a certain allele
What is Monohybrid inheritance
The inheritance of a single gene, with one dominant allele and one recessive allele
How are alleles written for monohybrid inheritance
- Upper case letters show the dominant allele
- Lower case letter show the recessive allele
- 2 Alleles written for diploid organisms
- Both are the same letter
- eg. Pea pod colour:
- G = Green
- g = Yellow
What does it mean if an organism is ‘Heterozygous’ for a particular characteristic
The organism has one dominant allele, and one recessive allele for the gene
- Eg. Pea pods (G = Green, g = Yellow)
- Heterozygous: Gg
- ∴ Phenotype = Green, as dominant allele is expressed
What does it mean if an organism is ‘Homozygous’ for a particular characteristic
The organism has 2 of the same allele for the gene
- Eg. Pea pods (G = Green, g = Yellow)
- Homozygous Dominant: GG
∴ Phenotype = Green
- Heterozygous Recessive: gg
∴ Phenotype = Yellow
How are genetic crosses shown on a Punnet square
- Each possible gamete from one parent is placed at the top of each column
- Each possible gamete from the other parent is placed at the end of each row
- ∴ All possible genetic crosses can be seen by combining the gametes of both parents in all possible ways
- Allows genotype and phenotype ratios to be determined (probability that off-spring will have each possible combination)
What are the genotype and phenotype ratios of the off-spring from a cross between two heterozygous parents in monohybrid inheritance (eg. Pea colour)
Gg x Gg
(Gametes for each parent: G, g)
- Genotype:
- GG : Gg : gg = 1:2:1
- Phenotype:
- Dominant : Recessive = 3:1
How do you determine the genotype of an organism with a dominant phenotype
(Could be Heterozygous or Homozygous dominant…)
Test Cross:
Cross the organism that has the Dominant phenotype (Unknown genotype) with an organism that has a Recessive phenotype (Recessive genotype)
- If organism with dominant phenotype is Homozygous dominant:
- All offspring will be heterozygous, as each will have one dominant allele from the homozygous dominant parent
- ∴ All off-spring will have dominant phenotype
- If organism with dominant phenotype is Heterozygous:
- Half of the off-spring will be heterozygous and half will be homozygous recessive, as half will have inherited a dominant allele from the heterozygous parent and half will have inherited a recessive allele from them
- ∴ Half of the off-spring will have dominant phenotype, and half will have recessive phenotype
Can also be used for dihybrid inheritance, just considering the traits separately as if for monohybrid inheritance
What is Dihybrid inheritance
The inheritance of two different genes, located on two different chromosomes
How are alleles written for dihybrid inheritance
- Upper case letters show the dominant allele
- Lower case letter show the recessive allele
- 2 alleles written for each gene, for diploid organisms (4 total in dihybrid inheritance)
- Different letters used for different genes
- eg. Pea pod ‘colour’ and ‘shape’:
- G = Green
- g = Yellow
- R = Round
- r = Wrinkled
What are the phenotype ratios of the off-spring from a cross between two parents heterozygous for both traits in dihybrid inheritance (eg. Pea colour and shape)
GgRr x GgRr
(Gametes for each parent: GR, Gr, gR, gr)
- Phenotype:
- Dominant & Dominant : Dominant & Recessive : Recessive & Dominant : Recessive & Recessive
= 9:3:3:1
- Dominant & Dominant : Dominant & Recessive : Recessive & Dominant : Recessive & Recessive
What is Mendel’s ‘Law of independent assortment’
Each member of a pair of alleles can combine randomly with any other when inherited
- First recognised as the observed phenotype ratios of the off-spring from a genetic cross matched that of the expected ratio
What is Codominance
Inheritance in which both alleles are expressed in the phenotype of the offspring (Both are equally dominant)
How are alleles written when they are codominant
- Gene is written as the base (eg. C = colour)
- Superscript shows the allele
- All alleles are capitals, to show they are equally dominant
- Each allele is a different letter
- eg. C^(R) = Red
- eg. C^(W) = White
What is the Phenotype of an organism that is heterozygous for an organism, with 2 codominant alleles
If an organism has 2 different, codominant alleles for the same gene, both traits are expressed.
- eg. Flower colour
- C^(R)C^(R) = Red
- C^(W)C^(W) = White
- Heterozygous: C^(R)C^(W) = Pink (mix of both traits)
What are the genotype and phenotype ratios of the off-spring from a cross between two heterozygous parents in codominant inheritance (eg. Flower colour)
C^(R)C^(W) x C^(R)C^(W)
(Gametes for each parent: C^(R), C^(W))
- Genotype:
- C^(R)C^(R) : C^(R)C^(W) : C^(W)C^(W) = 1:2:1
- Phenotype:
- Red : Pink : White = 1:2:1
What is inheritance involving ‘multiple alleles’
When a gene with more than two possible alleles is inherited
- However, there are still only 2 loci for each gene in diploid organisms
- eg. Gene for blood type
How are alleles written for inheritance involving ‘multiple alleles’
(eg. blood groups)
- Gene is written as the base (eg. I = Immunoglobin gene)
- Superscript shows the allele
- Alleles can be capital or lowercase to show if they are dominant or recessive
- If two dominant are present, mixed phenotype is seen like in codominance (eg. Blood group AB)
- Each allele is a different letter
- eg. I^(A) = Red blood cells have surface antigen A (dominant)
- eg. I^(B) = Red blood cells have surface antigen B (dominant)
- eg. I^(o) = Red blood cells have surface antigen o (recessive)
What is ‘Sex-linked’ inheritance
Inheritance of a Gene located on either the X or Y chromosome
What is the difference between the sex-chromosomes of Males and Females
Males = XY
Females = XX
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Section 1 - Biological molecules: 1. Biological molecules79
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Section 1 - Biological molecules: 2. Nucleic acids62
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Section 2 - Cells: 3. Cell structure74
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Section 2 - Cells: 4. Transport across cell membranes42
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Section 2 - Cells: 5. Cell recognition and the immune system50
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Section 3 - Organisms exchange substances with their environment: 6. Exchange80
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Section 3 - Organisms exchange substances with their environment: 7. Mass transport75
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Section 4 - Genetic information, variation and relationships between organisms: 8. DNA, genes and protein synthesis42
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Section 4 - Genetic information, variation and relationships between organisms: 9. Genetic diversity42
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Section 4 - Genetic information, variation and relationships between organisms: 10. Biodiversity82
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Section 5 - Energy transfer in and between organisms: 11. Photosynthesis34
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Section 5 - Energy transfer in and between organisms: 12. Respiration33
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Section 5 - Energy transfer in and between organisms: 13. Energy and Ecosystems31
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Section 6 - Organisms respond to changes in their environment: 14. Response to stimuli66
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Section 6 - Organisms respond to changes in their environment: 15. Nervous coordination and muscles59
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Section 6 - Organisms respond to changes in their environment: 16. Homeostasis58
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Section 7 - Genetics, populations, evolution and ecosystems: 17. Inherited change37
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Section 7 - Genetics, populations, evolution and ecosystems: 18 Populations and evolution32
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Section 7 - Genetics, populations, evolution and ecosystems: 19. Populations in ecosystems36
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Section 8 - The control of gene expression: 20. Gene expression56
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Section 8 - The control of gene expression: 21. Recombinant DNA technology44
