Inheritance (SCR) Flashcards
Inheritance as the fusion of haploid gametes to give a diploid zygote.
- Plants, animals and other eukaryotes that reproduce sexually pass genes to offspring in gametes. This is the basis of inheritance
- Male and female gametes have the same haploid number of chromosomes so male and female parents make an equal genetic contribution to their offspring
- As gametes are produced from diploid body cells, meiosis is required to halve the chromosome number
- Diploid body cells have two copies of each autosomal gene (genes located on a non-sex linked chromosome) Only one of each gene is passed on to offspring in the gamete
- Fusion of male and female gametes doubles the chromosome number, so the zygote is diploid, as are all body cells subsequently produced by mitosis
Why would you carry out genetic crosses in flowering plants?
Patterns of inheritance can be investigated by crossing different varieties of flowering plants such as peas. Pea plants produce both male and female gametes so can self-pollinate and therefore self-fertilise
How do you carry out a genetic cross in flowering plants?
- Cut off all the anthers from a flower on the plant that is intended to be the female parent. The anthers must be removed before they start to shed pollen, so the flower cannot self-pollinate.
- Enclose the flower in a paper bag to prevent insects or wind from transferring pollen to it.
- When the stigma of the flower is mature, transfer pollen to anthers on the intended male parent.
- Wait for the pollen to male gametes germinate and the male gametes to be carried down to the ovary in a pollen tube, where they will fertilise egg cells inside the ovules, resulting eventually in seeds which can be harvested
What are genotypes
“The genetic makeup of an organism”
Humans and other diploid organisms have two alleles of autosomal genes, one inherited from each parent. There could be two copies of one allele, or two different alleles. For example, for a gene with the alleles D and d, an individual could have DD, dd or Dd. Combinations of alleles such as these are known as genotypes.
what are alleles?
Alleles are different versions of the same gene
What is the phenotype?
The phenotype of an organism is its observable traits (characteristics). Phenotype includes structural traits such as whether hair is curly or straight and functional traits such as the ability to distinguish red and green colours.
Most phenotypic traits are due to the interaction between the genotype of an organism and the environment in which it exists, but there are some determined solely by genotype and some solely by environmental factors.
which phenotypes are determined by the genotype only?
- eye colour-brown or blue/ grey
- haemophilia-blood slow to clot
- ability to smell B-ionone-an odorant in violets.
which phenotypes are determined by genotype and the environment?
- height in humans
- autism- a personality trait
- diabetes- failure to regulate blood glucose concentration
which phenotypes/traits are determined by the environment only?
- scars due to surgery or wounds
- river blindness- due to parasitic worms in the eye
- body art such as tattoos or piercings
Who discovered that there were dominant and recessive alleles, and how?
Using pure-breeding varieties of pea plant, Gregor Mendel discovered a pattern of inheritance in which one allele of a gene is dominant and another allele is recessive.
what is the dominant allele?
Dominant alleles determine the phenotype in both individuals that are homozygous for the dominant allele (DD, for example) and in individuals that are heterozygous with one dominant and one recessive allele (Dd, for example).
what is the recessive allele?
Recessive alleles only determine the phenotype if an individual is homozygous with two recessive alleles (dd, for example. Mendel crossed two pure breeding varieties together that differed in a clear trait such as height (tall or dwarf). With each of the traits that Mendel tested, the Fl offspring all had the same phenotype as one of the two parents. For example offspring of a cross between tall and dwarf pea plants were all tall. When Mendel self-pollinated these Fl pea plants, the phenotype that was not seen in the Fl generation reappeared in 25% of the F2 pea plants.
What is phenotypic plasticity?
Organisms can respond to their environment by varying their patterns of gene expression and therefore their traits.
This is a form of adaptation, but it is reversible because genes have only been switched on or off, not changed into new alleles. It is known as phenotypic plasticity and is particularly useful if the environment a population inhabits is variable. For example, a person with pale skin may become darker-skinned if there is an increase in exposure to sunlight. A change in gene expression results in increased synthesis of the black pigment melanin in the skin. If the sunlight stimulus diminishes, gene expression reverts to its former pattern and the skin gradually becomes paler again, as the melanin concentration reduces. In some cases, phenotypic changes in traits cannot be reversed during the lifetime of the individual, but can when offspring are produced.
What is phenylkeronuria?
The genetic disease phenylketonuria (PKU) is due to a recessive allele of the gene that codes for the enzyme phenylalanine hydroxylase. This enzyme converts phenylalanine into tyrosine.
The PKU allele is recessive because a carrier with one
PKU allele can still produce functioning enzymes by expressing their normal allele. A person with two recessive PKU alleles does not produce functioning enzyme so phenylalanine accumulates in the body and there is tyrosine deficiency.
In excess, phenylalanine impairs brain development, leading to intellectual disability and mental disorders.
This can be prevented by screening for PKU at birth and giving affected children a diet low in phenylalanine.
Explain multiple alleles using the ABO blood group
The ABO blood group system in a human is an example of multiple alleles. One gene determines the ABO blood group of a person. There are 3 alleles of the gene Iᴬ, Iᴮ and i (Iᴼ). There are 4 different blood groups: A, B, AB, O.
- Iᴬ is dominant over i so people with either genotype IᴬIᴬ or Iᴬi are in blood group A
- Iᴮ is dominant over i, so people with either genotype IᴮIᴮ or Iᴮi are in blood group B
- i is recessive to Iᴬ and Iᴮ, so only people with genotype ii are in blood group O
- Iᴬ and Iᴮ are co-dominant so people with Iᴬ Iᴮ are in blood group AB
What is incomplete dominance?
Some pairs of alleles show incomplete dominance. The phenotype of a heterozygous individual is intermediate between the phenotypes of the two types of homozygotes. For example, if homozygous red-flowered plants are crossed with homozygous white-flowered plants, the heterozygous offspring all have pink flowers.
White flowers contain no red pigment. Pink flowers are intermediate because they contain some red pigment, but less than in a red flower. If two pink-flowered plants are crossed together, the ratio of flower colours in the offspring is 1 red: 2 pink: 1 white.
Explain co-dominance with reference to ABO blood group system
Some pairs of alleles show co-dominance. Homozygous individuals have a dual phenotype that is different to those of either of the two types of homozygotes.
The Iᴬ and Iᴮ alleles in the ABO blood group system are an example because blood group AB is a dual phenotype and is not intermediate between group A and group B
- Iᴬ causes the production of a specific glycoprotein in the plasma membrane of red blood cells that acts as an antigen
- Iᴮ causes production of another glycoprotein in the plasma membrane that acts as a different antigen
What are the two sex chromosomes?
Humans have 23 pairs of chromosomes in their body cells. Sex is determined by the 23rd pair of chromosomes. There are two types of sex chromosomes, X and Y.
- Females typically have two X chromosomes, so all female gametes have one X chromosome and all offspring inherit an X chromosome from their mother
- Males typically have one X and one Y chromosome, so male gametes either contain an X or a Y chromosome
- The sex of offspring is therefore determined by the sperm that fertilises the egg. A sperm with a Y chromosome makes the resulting child male, whereas an X-bearing sperm makes the child female
what does the Y chromosome do?
The Y chromosome is small with only about 55 genes, many of which are unique to the Y chromosome and are not needed in females. One key gene on the Y chromosome causes gonads in a human embryo to develop into testes. This gene is the testis determining factor (TDF). The developing testes in an embryo start to secrete testosterone, which causes the development of other organs of the male reproductive system.
The gonads develop into ovaries in an embryo without a Y chromosome and therefore no TDF gene. The embryonic ovaries start to secrete oestradiol, causing the development of the female reproductive system.
what does the X chromosome do?
The X chromosome is relatively large and has around 900 genes, many of which are essential in both males and females. All humans must therefore have at least one X chromosome. Because females have two copies of genes on the X chromosome and males only have one, the inheritance pattern differs in males and females.
what are sex-linked genetic disorders?
Sex-linked genetic disorders are due to genes located on the X chromosome. Most sex-linked disorders are due to a recessive allele of the gene. Males only have one copy of genes on the X chromosome, so they have the disorder if this one copy is the recessive allele. Females are much less likely to be affected because as long as one of their two X chromosomes carries the dominant allele, they are unaffected.
What is haemophilia?
Haemophilia is an example of this pattern of sex-linked inheritance. People with this disorder, usually males, either lack or have a defective form of factor VIII. This protein is a clotting factor that normally circulates in the blood. Cuts and other wounds bleed for much longer than normal in people with Haemophilia.
The gene for factor VIII is located on the X chromosome. The allele that causes haemophilia is recessive. The frequency of the allele is about 1 in 10,000. This is therefore the frequency of the disease in boys. Females only develop haemophilia in the very rare cases where both of their X chromosomes carry the recessive allele, but much more frequently they are carriers of the haemophilia allele.
how do you show sex-linked inheritance on a Punnett square?
The pattern of sex-linked inheritance can be shown using Punnett grids. The alleles should always be shown as a superscript letter on the letter X to represent the X chromosome. The Y chromosome should also be shown although it does not carry an allele of the gene.
what is Huntingtons disease?
The disease causes a wide range of symptoms and people aren’t all similarly affected. Movement problems include trouble with coordination, and involuntary jerking or twitching movements (chorea), and, as the disease progresses, patients have problems with swallowing. Cognitive and behavioural symptoms include difficulties in learning new information, depression, personality changes, mood disorders and hallucinations. The age of onset is variable.
