Define gene
A sequence of bases on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic
Key Terms
Define gene
A sequence of bases on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic
Define Genotype
The genetic constitution of an organism
(i.e. the alleles an organism has)
Define Phenotype
The expression of the genetic constitution and its interaction with the environment
(i.e. an organism’s characteristics)
Define Allele
A different version of a gene
Explain what is meant by a recessive allele
Only expressed in phenotype when 2 alleles are present
Explain what is meant by a dominant allele
Is always expressed in the phenotype
Related Flashcard Decks
Study Tips
- Press F to enter focus mode for distraction-free studying
- Review cards regularly to improve retention
- Try to recall the answer before flipping the card
- Share this deck with friends to study together
| Term | Definition |
|---|---|
Define gene | A sequence of bases on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic |
Define Genotype | The genetic constitution of an organism (i.e. the alleles an organism has) |
Define Phenotype | The expression of the genetic constitution and its interaction with the environment (i.e. an organism’s characteristics) |
Define Allele | A different version of a gene |
Explain what is meant by a recessive allele | Only expressed in phenotype when 2 alleles are present |
Explain what is meant by a dominant allele | Is always expressed in the phenotype |
Explain what is meant by co-dominant alleles | Both alleles are expressed in the phenotype |
Define Homozygous | An organism that carries 2 copies of the same allele |
Define Heterozygous | An organism that carriers 2 different alleles |
Define Carrier | Person carrying an allele which isn’t expressed in phenotype but can be passed on to offspring |
Define Locus | Fixed position of gene on a chromosome (Alleles of gene are found at same locus on each chromosome in a pair) |
Define Autosome | Any chromosome that is not a sex chromosome |
Define Allosome | Sex chromosome |
What is monohybrid inheritance? | Inheritance of a characteristic controlled by a single gene |
Draw a monohybrid cross showing how wing length is inherited in fruit flies when the parent’s genotypes are NN x nn. Include the phenotypes and phenotypic ratio. N - normal wings allele n - vestigial (little) wings allele | Parents: NN (normal wings) × nn (vestigial wings) |
Draw genetic diagram when the paren’ts genotypes are HNHS x HNHS (i.e. crossing 2 parents with sickle-cell trait (heterozygous)) HN = normal haemoglobin HS = sickle haemoglobin | Parents: HNHS × HNHS |
In the ABO blood group system in humans there are 3 alleles for blood type: IO is the allele for blood group O IA is the allele for blood group A IB is the allele for blood group B Allele IO is recessive. Alleles IA and IB are codominant - people with genotype IAIB will have blood group AB. Draw a genetic diagram that shows a cross between a heterozygous person with blood group A and a heterozygous person with blood group B. |
|
What do dihybrid crosses show? | Show how 2 genes are inherited are once |
Draw a dihybrid cross diagram for when the both parents’ gentotype is RrYy. Include the phenotype and the phenotypic ratio. R - round seed r - wrinkled seed Y - yellow seed y - green seed | Parents: RrYy × RrYy |
When are characteristics said to be sex-linked? | When the allele that codes for a characteristic is located on a sex chromosome |
Most genes on sex chromosomes are only carried on ___ chromosomes | X (X-linked genes) |
Explain why males are more likely than females to show recessive phenotypes for genes that are sex-linked | Males only have 1 X = 1 allele for sex-linked genes ∵ have only 1 copy, express characteristic of allele even if its recessive |
Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X chromosome Given that the female parent is a carrier and the male is unaffected, draw monohybrid diagram (n = faulty allele for a colour vision) | Female parent: XᴺXⁿ (carrier), Male parent: XᴺY (unaffected) |
What are autosomal genes? | Genes located on the autosomes |
Genes on the same autosome are said to be ____ | linked |
Why are autosomal genes said to be linked? | ∵ they're on the same autosome, they'll stay together in independent segregation of chromosomes in meiosis I And their alleles will be passed on to offspring together (Only doesn't occur if crossing over splits them up 1st) |
The closer together 2 genes are on autosome, the more … | closely they're linked |
Why is that the more closer together 2 genes are on autosome, the more closely they're linked? | ∵ reduces the chance of genes being separated during crossing over |
What happens to the phenotypic ratio expected in offspring when 2 gene are autosomally linked? | It changes - won't get the phenotypic ratio you expect in offspring of cross e.g. Dihybrid cross between 2 heterozygous parents = 9:3:3:1 ratio in offspring Instead phenotypic ratio more likely to be like monohybrid cross between heterozygous parents (3:1) |
Why is the phenotypic ratio different when 2 genes are autosomally linked? | ∵ higher proportion of offspring will their parents' (heterozygous) genotype and phenotype |
Explain the difference in the phenotypic ratios | NR and nr linked (in NnRr parent) NnRr parent produces mainly NR and nr gametes So fewer Nnrr and nnRr Crossing over produces some Nr and nR gametes |
What is epistasis? | When an allele of 1 gene masks (blocks) the expression of the alleles of other genes |
Name the 3 types of epistasis | Recessive epistasis Dominant epistasis Complementary epistasis |
Describe dominant epistasis | Having at least one copy of the dominant epistatic allele masks (blocks) the expression of the other gene |
Dominant Epistasis State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent | 12 : 3 : 1 dominant epistatic : recessive epistatic dominant other : recessive both |
Squash colour is controlled by 2 genes - the colour espistatic gene (W/w) and the yellow gene (Y/y). The no-colour, white allele (W) is dominant over the coloured allele (w). The yellow gene has the dominant yellow allele (Y) and the recessive green allele (y). Draw a genetic digram crossing WwYy with WwYy. Include the phenotypes and phenotypic ratio. | Parents: WwYy × WwYy |
Describe recessive epistasis | Having 2 copies of the recessive epistatic allele masks (blocks) the expression of the other gene |
Recessive Epistasis State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent | 9 : 3 : 4 dominant both : dominant epistatic recessive other : recessive epistatic |
Flower pigment in a plant is controlled by 2 genes. Gene 1 codes for a yellow pigment (Y = dominant yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R = dominant yellow allele). Cross YyRr x YyRr. Include phenotypes and phenotypic ratios. | Parents: YyRr × YyRr |
When do you use the chi-squared (χ2) test? | Used to see if results of experiment support a theory |
State the chi-squared formula | O = Observed result E = Expected result |
To find if there's a significant difference between your observed and expected results, you need to compare χ2 value to ____ ____ | critical value |
If χ2 ≥ critical value, there's a | Significant difference (Something other than chance causing difference) Null hypothesis can be rejected |
If χ2 \< critical value, there's … | NO significant difference Accept null hypothesis |
Describe how you work out the degrees of freedom | Number of classes (no. of phenotypes) - 1 e.g. 2 - 1 = 1 |
How do multiple alleles of a gene arise? (2) | Due to mutations Which occur in different positions in the gene |
At extremities (e.g. legs/arms) temperature is _____ | lower |
Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is recessive (2) | 3 & 4 produce 9/11 who are affected Both 3 and 4 are heterozygous (carry recessive allele) |
Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is not on the X chromosome (2) | 11 is affected, 3 is not (If on X) 11 / affected female would not receive the recessive allele on X chromosome / Xt from 3 / father |
Explain why sex-linked conditions are more common in males than in females (3) | Males have XY chromosomes and females have XX chromosomes Recessive allele on X chromosome has no (equivalent) allele on Y chromosome Male needs to inherit one recessive allele whereas female needs to inherit two |
Suggest one reason why observed ratios are often not the same as expected ratios (1) | Due to chance Fertilisation is random |
How is variation genetically controlled? (1) | Polygenes / several genes |
Extension: State the genotype of the heterozygous parent using the correct notation | The genotype of the heterozygous parent is YyRr. |