6.1.2 (b) Flashcards
What are the rules for using a genetic diagram?
- State the phenotype of both parents
- State the genotype of both parents
- Use a capital letter for a dominant allele
- Use the same letter in simple form for a recessive allele
- State the gametes and circle
- Use a Punnett Square to show random fusion of gametes during fertilisation
- Ensure the gametes are labelled
- Write out the genotypes and corresponding phenotypes as a ratio/percentage
What is a monogenic inheritance?
Show an example of a genetic cross of monogenic inheritance
Monogenic Inheritance: Involving one gene
F1 – 1st filial (generation)
F2 – 2nd filial (generation)
What is codominance and incomplete dominance?
Codominance: Where both alleles of the parents are expressed to the phenotype of the offspring due to the equal dominance of the alleles
Incomplete Dominance: Both are alleles are expressed but the phenotype is mixed
What are multiple alleles?
- Occurs when there are >2 versions of a gene
- Each homologous chromosome can only contain 2 copies of the gene
- E.g. blood type
- 3 alleles: IA and IB which are codominant (express both alleles expressed in phenotype), IO is recessive to both IA and IB
Describe the characteristics of human sex chromosomes
- In determining sex, humans have 23 pairs of chromosomes which vary in shape and size
- 22 of the pairs are the same but the 23rd pair is the sex chromosomes (different sizes)
- Females: XX; Males: XY
- X chromosome is large, has many genes and no involvement in sexual development
- Y chromosome is small, almost no genetic info, carries the gene causing the embryo to develop a male
What are sex-linked genes?
Sex-linked genes: Genes carried on sex chromosomes
- Since the Y chromosome is so much smaller than the X one, males will only have 1 copy of certain genes
- So even if a characteristic is recessive, and on the X chromosome, the characteristic will occur more frequently in males
- Such as colour blindness, Duchenne’s Muscular Dystrophy, Haemophilia
in a genetic cross the Y chromosome exists as just ‘Y’ with no superscript allele
What are linked and unlinked genes?
There are 23 pairs of chromosomes, 1 pair is the sex chromosomes
- The other 22 pairs are homologous and known as autosomes
- Autosomes: Chromosomes not concerned with sex determination
- Linked Genes: Genes that appear on the same chromosome
What is autosomal linkage?
Autosomal linkage: If 2 or more genes are located on the same chromosome
- Genes that are located on separate homologous chromosomes are independently assorted into gametes during crossing over in meiosis
- In meiosis, during crossing over, alleles cross over giving recombinant chromatids and subsequently recombinant offspring
- The closer the genes are on a chromosome, the less likely they are to be separated during crossing over – fewer recombinant chromatids and offspring – meaning the linked genes are likely to be inherited and expressed together
What are the common phenotypic ratios?
- For a cross between a homozygous individual of alleles A and B (AABB) and a 2nd individual homozygous for a and b (aabb) the offspring in F1 will be heterozygous (AaBb), when you genetic cross 2 heterozygous individuals, with no linkage the ratio is always 9:3:3:1.
- If you cross the 2 heterozygous individuals (with linkage but no cross-overs) the ratio is always 3:1
What is epistasis?
Epistasis: Is the interaction between 2 non-linked genes (2 genes on 2 separate chromosomes) causing one gene to mask the expression of the other in the phenotype.
- Epistatic genes can work antagonistically (against one another) or complementarily
What is antagonistic epistasis?
- When a gene suppresses another gene, the gene doing the suppressing is called the epistatic gene
- The gene being suppressed is the hypostatic gene
- Two types of antagonistic epistasis – recessive; dominant
What is recessive antagonistic epistasis?
And the phenotypic ratios observed
- Occurs when the presence of two copies of the recessive allele at the 1st locus prevents the expression of another allele at the 2nd locus
- In the genetic cross, the aa is the recessive epistatic gene, it causes white mice, regardless of BB, Bb or bb.
The ratio of phenotypes observes will be a modified form of 9:3:4
What is dominant antagonistic epistasis?
Dominant Antagonistic Epistasis
- Occurs when the expression of the dominant allele of the epistatic gene masks the expression of the hypostatic gene
- If there are alleles A/a, the epistatic gene can have 1 or 2 dominant alleles, e.g Aa, AA
