Unit 2.4 Variation and Inheritance Flashcards
Species
Members of a species can interbreed to produce fertile offspring.
They are similar but not identical to each other.
They show variation as a result of sexual reproduction.
Continuous variation
Controlled by many genes (polygenic).
Characteristics controlled by many genes show a range of values, eg. height, leaf size
There are no distinct groups.
How is data showing continuous variation displayed?
Histogram or line graph
Discrete (discontinuous) variation
Characteristics are controlled by a single gene and fall into distinct groups.
eg. tongue rolling - you are a roller, or a non-roller and nothing in between.
Other examples include blood type and hitchhikers thumb.
How is data showing discrete variation displayed?
Bar chart
Normal distribution
The pattern produced when you plot a histogram of data showing continuous variation. eg. height
Most values are average, with few extremes.
The curve is ‘bell shaped’.
Monohybrid cross
Used to study the inheritance of characteristics that are controlled by a single gene and show discrete variation.
Alleles from 2 parents are combined and are used to predict the characteristics of the offspring.
Gene
A section of DNA that carries the instructions for building a protein.
Genes are found on chromosomes.
Alleles
The different versions of a gene.
They do the same job (eg. eye colour gene) but have a slightly different base sequence, so produce a slightly different protein.
Alleles for the eye colour gene are blue, green and brown.
Homologous chromosomes
The 2 matching chromosomes in a pair.
One came from each parent.
They carry the same gene at the same location, but each one may have a different allele.
Genotype
A 2 letter code describing which alleles have been inherited form each parent.
eg. BB indicates that a B allele came from each parent; Bb indicates that B came from one parent and b from the other.
Alleles - capital letters
A capital letter indicates a dominant allele. eg. B = brown
Alleles - lower case letters
A lower case letter indicates a recessive allele. eg. b = blue
Homozygous
If the 2 alleles in a homologous pair are the same, the organism is said to be homozygous.
eg. BB or bb
Heterozygous
If the 2 alleles in a homologous pair are different, the organism is said to be heterozygous.
eg. Bb
The characteristic that is present depends on which allele is dominant.
Phenotype
Describes how a gene is expressed.
It is often (but not always) appearance.
eg. blue eyes, brown fur, blood group A.
Phenotype is determined by 2 alleles working together. The dominant one is expressed, the recessive one may be present but hidden.
F1 generation
The 1st generation of offspring, produced by combining the alleles of 2 homozygous parents (P)
F2 generation
The offspring produced when the alleles of 2 members of the F1 generation are combined.
Punnett square
A grid used to show the results of a monohybrid cross.
Phenotype ratio
Phenotype ratio shows the predicted proportion of each phenotype that should be produced eg. 3 purple : 1 white flowered.
This ratio is not always achieved due to random fertilisation and different survival rates of offspring.
Genotype ratio
Genotype ratio shows the predicted proportion of each genotype that should produced eg. 1 BB : 2 Bb : 1 bb
This ratio is not always achieved due to random fertilisation and different survival rates of offspring.
Backcross
Used to identify whether an original parent is homozygous or heterozygous.
If the unknown parent is crossed with another individual of known genotype (homozygous recessive) then offspring ratios can be used to identify the unknown parent.
Family trees
Used to follow the inheritance of a characteristic between generations, by identifying the individuals that are homozygous recessive (bb).
Genetic counselling
Family trees are used to advise people who are concerned about passing on an inherited disease to their children.
Carriers may not suffer from a disease if it is recessive, but it can be inherited.
eg. cystic fibrosis,
Some genetic diseases have dominant alleles eg. Huntington’s disease.
