Inheritance Flashcards
What is monohybrid inheritance?
Inheritance of a single gene
What does pure-breeding mean and how does it come about?
Pure-breeding = homozygous for a particular characteristic.
Organisms with the same phenotype are consistently bred together so all offspring have this phenotype and they are homozygous for the gene.
Monohybrid inheritance: F1 generation when 2 pure breeding organisms are crossed - genotypes and phenotypes
Phenotype: all dominant
Genotype: all heterozygous
Monohybrid inheritance: F2 generation when F1 heterozygous offspring breeding are crossed - genotypes and phenotypes
Phenotype: 3:1 dominant to recessive
Genotype: 1 homozygous dominant: 2 heterozygous: 1 homozygous recessive
Why are the actual outcomes of genetic crosses different to the predicted?
due to statistical error: chance determines which gametes fuse with which.
How can you make sure the actual results are close to the theoretical ones when carrying out an crossing experiment?
By using a large sample size
What is dihybrid inheritance?
how two characteristics, determined by two different genes located on different chromosomes are inherited.
General dihybrid cross F1
Parents: XXYY and xxyy
Offspring: ?
all XxYy
all have phenotype of dominant for both genes
Why is the allele for one gene independent of the other in dihybrid inheritance?
As the genes are found on different chromosomes, and chromosomes arrange themselves at random during meiosis and independent segregation occurs.
Fertilisation is also random, so any of the 4 types of gametes can combine.
Types of gamete for an organism with genotype XxYy
XY, Xy, xY, xy
Dihybrid inheritance: theoretical ratio of phenotypes for the F2 generation - cross between 2 XxYy individuals.
9:3:3:1
Mendel’s law of independent assortment
each member of a pair of alleles may combine randomly with either of another pair.
Codominance
where 2 different alleles of a single gene trait are expressed in the phenotype - instead of one allele being dominant and the other recessive.
Xx
which letter represents the dominant allele and which the recessive
X = dominant x = recessive
How are codominant alleles represented?
Different letters are used to represent the alleles, as superscripts to a letter which represents the gene.
Why can’t upper and lowercase letters be used to represent codominant genes?
As this would imply that one gene is dominant.
Linked genes
genes that are inherited together, i.e. on the same chromosome, so if one gene is inherited, the linked one is too.
Unlinked genes
genes that are inherited separately - e.g. on different chromosomes, due to independent segregation.
Why can 4 different combinations of alleles in gametes arise with dihybrid inheritance?
Meiosis separates homologous chromosomes and alleles.
Independent segregation means you can get any combination of the 2 alleles an a gamete.
Multiple alleles
A gene may have more than 2 alleles, so more different combinations and phenotypes can arise.
Only 2 alleles can be present at one time as there are only 2 homologous chromosomes.
Example of multiple alleles
the human ABO blood system
Sex-linkage
inheritance of a gene located on one of the sex chromosomes
Autosome
any one of the 22 pairs of chromosomes which are not sex chromosomes.
Gene
A sequence of bases on a DNA molecule that codes for a protein which results in a characteristic.
Allele
A different version of the same gene. Most organisms have 2 alleles of each gene, one on each homologous chromosome.
Genotype
The genetic constitution of an organism - all the alleles it has
Phenotype
The expression of an organism’s genetic constitution and its interaction with the environment, an organism’s observable or biochemical characteristics.
Dominant
An allele which is expressed in the phenotype even when only one copy is present.
Represented with a capital letter.
Recessive
An allele which is only expressed in the phenotype if 2 copies are present.
Homozygote
An organism that carries 2 copies of the same allele, e.g. BB or bb
Heterozygote
An organism that carries 2 different alleles of the same gene, e.g. Bb
Carrier
An organism carrying an allele which is not expressed in the phenotype but can be passed on to offspring.
Why are males more likely to express recessive phenotypes?
as the X chromosome is much longer than the Y and carries more genes, so most of the X chromosome doesn’t have an equivalent homologous portion of the Y chromosome.
Males only have 1 X chromosome so often only have 1 allele for sex-linked genes. This allele is always expressed, even if it is recessive.
Autosomal linkage
Inheritance of 2+ genes which are carried on the same autosome.
Why are females less likely to be colour-blind than males?
As the condition is X-linked/ gene is found on the X chromosome, so females need 2 recessive alleles but males only need 1.
Epistasis
Where different genes control the same characteristic, and the expression of 1 gene affects the expression of another.
Examples of epistasis
Metabolic pathways - stages controlled by enzymes
Fur colour and fur length
What is the chi squared test used to test?
whether deviation between theoretical and experimental ratios/ outcomes for genetic crosses are significant or not.
Why are the ratios different to dihybrid inheritance when genes are linked? e.g. cross between 2 GgNn individuals,
Genes on same chromosome so GgNn individual produces mainly GN and gn gametes. Crossing over produces a few Gn and gN gametes, so there are fewer Ggnn and ggNn individuals.
What type of data is the chi squared test used for?
categorical
What type of epistatic allele would give these ratios in a dihybrid cross?
9: 3:4
12: 3:1
recessive
dominant
