Inheritance Flashcards
Dominant alleles
- ones which always expressed when present. They stop recessive alleles from presenting. Shown in genetic diagrams with a capital letter.
Recessive alleles
- these are only expressed when there is no dominant allele of the gene present; a person must have 2 recessive alleles for that characteristic to show. Represented in diagrams with a lower-case letter.
Some characteristics are controlled by a single gene, but most are controlled by many genes interacting. We call these polygenes, or ‘polygenic inheritance’. Can you think of any characteristics that might be controlled by more than just 1 gene?
- Natural skin and hair colour, height, eye colour, risk of disease, intelligence, blood pressure, bipolar disorder
Genotype
- the combination of alleles present. Each characteristic is coded for by at least 2 alleles, one from each parent. The dominant allele always has a capital letter, and you use the lowercase version of the same letter to show the recessive allele
Phenotype
- the actual characteristic expressed by a person’s genes (observable features of an organism). For example, brown hair, blue eyes, tongue-roller etc
Homozygous
- an individual has two identical alleles of a particular gene. They are described as ‘homozygous dominant’ or ‘homozygous recessive’. Two identical homozygous individuals that breed together will be pure-breeding.
Heterozygous
- individual has two different alleles of a particular gene. A heterozygous individual will not be pure-breeding
What do punnett squares do?
- predict the probability of certain phenotypes in the offspring
monohybrid inheritance
- inheritance of a single gene, controlled by different alleles
Law of Segregation
- In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair can be present in a single gamete.
phenotypic ratio
- ratio of all the possible phenotypes
Pedigree
- chart of the genetic history of a family over several generations
What are males represented as in pedigree charts?
- squares
What are females represented as in pedigree charts?
- circles
What do the shaded symbols represent in a pedigree chart?
- individual affected by a condition
What does a horizontal line between a man and a woman represent in a pedigree chart?
- mating
What does an unshaded symbol mean in a pedigree chart?
- unaffected by condition
What are resulting children represented as in a pedigree chart?
- offshoots to the horizontal line
What are generations labelled as in a pedigree chart?
- Roman numerals
What are individuals labelled as in a pedigree chart?
- according to age (oldest on left)
State what is meant by the term gene.
- section/length of DNA or chromosome that codes for a protein
Scientists observe that chickens have either normal feathers or silkie feathers. However, they have a wide range of different heights. Explain why there is a wider range of variation in height than in feather type.
- Feather is discontinuous , height is continuous, height is polygenic, height depends on several genes, height may have environmental effects, feather structure due to one gene only.
The inheritance of body colour in fruit flies was investigated. Two fruit flies with grey bodies were crossed. Of the offspring, 152 had grey bodies and 48 had black bodies. Explain why a statistical test should be applied to the data obtained in this investigation.
- To determine the probability; Of the
results being due to chance;
Researchers investigated some characteristics of people from different parts of England. In the north of England they selected 200 people and recorded their phenotypes for three different characteristics.
Their results are shown in the figure below. For each characteristic (tongue rolling / thumb) suggest one reason why the numbers do not add up to 200.
- Some embarrassed / not willing to show tongue / cannot tell; could not decide if thumb straight or not / thumb bend is subjective;
Why is it called a homologous pair of chromosomes?
- similar in size and shape, and they carry genes controlling the same features at the same positions on each
Describe a test cross
- to find outs genotype carry out test cross which means breeding the test subject with one that is homozygous recessive
What is the phenotypic ratio of second filial (F2 generation)?
- 3:1 (dominant : recessive)
What is the first filial (F1 generation)?
- Offspring of a monohybrid cross between homozygous dominant and homozygous recessive parents
What is the second filial (F2 generation)?
- offspring of F1 intercross
What is dihybrid inheritance?
- The inheritance of two distinct traits, controlled by two separate genes, located on different chromosomes
What is the Law of Independent Assortment?
- Each member of a pair of alleles may combine randomly with either of another pair
What is the theoretical ratio expected for a dihybrid cross between two heterozygous individuals (for both traits)?
- 9:3:3:1
Dihybrid questions involving chi squared
- 9:3:3:1 ratio…so you need to calculate the right proportions, 9+3+3+1 = 16… Multiply 16 by x to get x
Which chromosomes are called the sex chromosomes?
- pair 23
Sex linked characteristics
- where the gene responsible is located on one of the chromosomes
- certain disease traits are more common in one sex
What are some examples of sex linked diseases?
- red-green colourblindness, duchenne muscular dystrophy
What alleles are sex linked conditions caused by?
- recessive alleles
How are sex linked alleles represented?
- X and Y chromosomes written in caps with the disease related alleles written in superscript (above them)
Explain X^r X^r and X^r Y
- female would have 1 X with the recessive gene, but the normal allele on the other X chromosome would mask its effect, and so she is a CARRIER
- female is affected, as both X chromosomes carry the recessive allele
- male is affected, as he only has the one X chromosome. Such males are hemizygous – they only have one allele for a particular characteristic
Hemizygous
- only have one allele for a particular characteristic
Codominance (codominant alleles)
- both alleles in a heterozygous organism are expressed ‘equally and independently’ in the phenotype
How are codomimant alleles presented in a Punnett square?
- assign letters to the codominant alleles, and show them superscript, on a designated letter to represent the trait
Suggest how the farmer who owns these cattle could control the breeding so that the herd ultimately consisted of red coloured cattle only?
- To produce only red cattle, you only breed those that carried the red allele (pure-breeding red cattle) Eventually, the allele for white hairs would be removed from the population
What are immunoglobulins?
- antibodies
When is the immunoglobulin initial used in a Punnett square?
- for blood types
What are the four pedigree patterns?
- Autosomal dominant
- Autosomal recessive
- X linked recessive
- X linked dominant
What is autosomal dominant pattern?
- If both parents are affected and an offspring
is unaffected, the trait must be dominant (parents are both heterozygous). All affected individuals must have at least one affected parent
Autosome - chromosomes that are not sex chromosomes (i.e, any from pairs 1-22) - If both parents are unaffected, all offspring must be unaffected (homozygous recessive)
- cannot be recessive as two affected parents could not have unaffected offspring so parents must be heterozygous
Autosome
- chromosomes that are not sex chromosomes (i.e, any from pairs 1-22)
What is autosomal recessive pattern?
- Both parents are unaffected and offspring is affected, the trait must be recessive (parents are heterozygous carriers)
- If both parents show a trait, all offspring must also exhibit the trait (homozygous recessive)
- cannot be dominant as two unaffected parents could not have an affected offspring so parents must be heterozygous
What is X linked dominant pattern?
- If a male shows a trait, so too must all daughters as well as his mother
- An unaffected mother cannot have affected sons (or an affected father - remember, sons get the ‘Y’ chromosome from their fathers!)
- X-linked dominant traits tend to be more common in females
- sex linkage cannot be confirmed
- 100% incidence of affected daughters from an affected father suggests X linked dominance
What is X linked recessive pattern?
- If a female shows a trait, so too must all sons as well as her father
- An unaffected mother can have affected sons if she is a carrier (heterozygous)
- X-linked recessive traits tend to be more common in males
- sex linkage cannot be confirmed
- 100% incidence of affected sons from an affected mother suggests X linked recessive
Suggest why haemophilia was not present in the British Royal Family of Queen Elizabeth 2nd, Prince Phillip and their children?
- ancestors (Edward 7th and Victoria) did not have, nor carry any alleles for the condition
Give evidence from the chart that shows that haemophilia is i) sex-linked, ii) recessive
i) Only occurs in males and not females
ii) Parents without disease have affected children
Draw autosomal dominant pattern
Draw autosomal recessive pattern
Draw X linked dominant pattern
Draw X linked recessive pattern
What is autosomal linkage?
- alleles on the same chromosome inherited together and are said to be linked, forming a linkage group
Why do linked genes not assort independently?
- linked on the same chromosome
When does recombination of linked genes occur?
- when crossing over occurs
The allele for purple kernels (C) is dominant to the allele for white kernels (c). The allele for starchy kernels (W) is dominant to the allele for waxy kernels (w). Show the genotypes of the F1 offspring of a cross between two homozygous parents for both traits
Parent phenotypes: Purple starchy x White waxy
Parent genotypes: CCWW x ccww
Gametes produced: CW cw
F1 genotypes: CcWw
F1 phenotypes: Purple and starchy
Parent phenotypes: Purple starchy x Purple starchy
Parent genotypes: CcWw x CcWw
Gametes produced: CW cw CW cw
F2 genotypes: CCWW CcWw CcWw ccww
F2 phenotypes: 3 purple starchy: 1 white waxy
What does it mean if the observed ratio is different to the expected ratio?
- autosomal linkage
What is the expected ratio in an F1 intercross when original parents were both homozygous dominant and recessive?
- 9:3:3:1
parents we crossed weren’t both homozygous dominant and recessive so what we ‘expect’ comes purely from our punnett square, which was 1:1:1:1. So why did we get 1.7:1.1:1:1.8?
- The GB alleles and the gb alleles in the GgBb parent may have been linked.
- would mean that the GgBb parent produced mostly GB and gb gametes - this would make GgBb and ggbb genotypes more common in the offspring. - As a result, a higher proportion of the offspring would have their parent’s phenotypes, instead of the even split of phenotypes predicted.
Define epistasis
- The interaction of different gene loci so that one gene locus masks or suppresses the expression of another. This can reduce phenotypic variation.
How does epistasis work?
1) The genes work antagonistically against each other (directly or indirectly) resulting in masking
2) They work in a complimentary fashion
Epistatic gene
- A gene that interferes with or masks the expression of another gene. Epistatic genes are sometimes called inhibiting genes.
Hypostatic gene
- A gene whose expression is affected by an epistatic gene.
Recessive epistasis
- The homozygous presence of a recessive allele may prevent the expression of another allele at a second locus
Example of recessive epistasis
- Inheritance of flower colour in Salvia
Dominant epistasis
- A dominant allele at one gene locus masks the expression of the alleles at a second gene locus
Example of dominant epistasis
- Feather colour in chickens
Complementary epistasis
- At least one dominant allele for both gene loci present can result in them complementing one another – e.g. dominant genes produce functional enzymes A and B; A allows for production of an intermediate red pigment; enzyme B allows for the red pigment to be converted into a final blue pigmented product.
Example of complimentary epistasis
- Cross white flowered sweet peas to produce purple offspring
recessive epistasis ratio
- 9:3:4
Dominant epistasis ratio
- 12:3:1 / 13:3
Complementary epistasis ratio
- 9:7
In corn plants, a dominant allele I inhibits kernel color, while the recessive allele i permits color when homozygous. At a different locus, the dominant gene P causes purple kernel color, while the homozygous recessive pp causes red kernels. If plants heterozygous for both alleles are crossed, what will be the phenotypic ratio of the F1 generation?
I = no colour
i = colour (homozygous) P = purple
p = red (homozygous)
- Parental Phenotypes: Colourless x Colourless
- Parental Genotypes: IiPp x IiPp
- Gametes (for both): IP Ip iP ip
- After Punnett square: Phenotypic ratio of - 12 colourless: 3 purple: 1 red
Coat colour in mice may be agouti (greyish), black or albino. The gene for agouti has 2 alleles, ‘A/a’. Allele ‘a’ is a mutation; when homozygous, it produces a black coat. A gene ‘B/b’ at a separate locus controls the formation of pigment. Those who are ‘bb’ cannot produce pigment and are albino. Which epistasis would you see when you cross two agouti individuals ‘AaBb’?
A = agouti
a = black (homozygous) B = pigment produced b = no pigment
- Parental Phenotypes: Agouti x Agouti
- Parental Genotypes: AaBb x AaBb
- Gametes (for both): AB Ab aB ab
- After Punnett square: Phenotypic ratio of – 9 agouti : 3 black: 4 albino
