3.4 Inheritance Flashcards
Gregor Johann Mendel
- Discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed
Law of Segregation
Mendel’s first law
When gametes form, alleles are separated so that each gamete carries only one allele for each gene.
- Gametes are haploid so contain only one allele of each gene.
- The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
Law of independent assortment
Mendel’s second law
Alleles of different genes assort independently of one another during gene formation.
Allele
Different forms of the same gene
Genotype
The two alleles an individual has for a gene
Phenotype
The trait (observable characteristics) caused by the genotype
Dominant
An allele which masks the effects of another (recessive) allele
- Shown with a capital letter
Recessive
An allele which is masked by a dominant allele, in terms of the trait that is expressed.
- Shown with a lowercase letter
Homozygous
2 identical alleles (e.g. tt)
Heterozygous
Two different alleles (e.g. Tt)
Zygotes
When a sperm and ovum fuse in fertilisation , they form a single diploid cell called a zygote
Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.
Co-dominanance
Alleles that jointly affect the phenotype when present together
- Both are independently and equally expressed
Incomplete dominance
Alleles that jointly affect the phenotype when present together
- The phenotype of the heterozygous is a blend of the dominant and recessive characteristics
Example: the pink flower resulting from the red and white flowers
Monohybrid cross
learn punnet square
Step 1: two homozygous parents produce heterozygous offspring, this tells us the dominant gene
Step 2: the heterozygous offspring are crossed giving us a 3:1 ratio dominant:recessive
Sex linked diseases
- Sex-linked traits are traits associated with the X chromosome because the X chromosome carries more genetic information.
- We are always assuming that the gene is carried on the X chromosome (NOT ON Y CHROMOSOME)
- Males only have one allele
- Females have two alleles, if the allele is recessive they can be carriers of sex linked diseases
Note: alleles carried on X chromosomes should be shown as superscript letters on an upper case X, such as Xh.
Cystic fibrosis
Autosomal recessive disease
- Autosomal recessive disease
- Caused by a mutation in a gene on chromosome 5
- The person has to carry both recessive alleles to express the diseased phenotype
- People with CF produce abnormally thick and sticky mucus in their lungs and airways
- They are more likely to get respiratory infections
- Daily physiotherapy helps to relieve congestion, while antibiotics can fight infection
- The disease blocks tubes that take enzymes to the gut meaning food is not digested properly, leaving the person short of essential nutrients.
Huntington’s disease
Autosomal dominant disorder
- Autosomal dominant disease
- The person only needs one dominant gene to express the diseased phenotype
- Caused by a mutation on a gene on chromosome 4
- Alters the structure of an important protein called the Huntington protein that supports brain activity
- Leads to neurodegeneration
- The symptoms usually develop in middle age, and include tremors, mood changes, memory loss and the inability to concentrate
Sex linked inheritance examples
- Red green colour blindness
- Haemophilia: protein needed for blood clotting is not made
Both are sex-linked recessive disorders
- This means that females are less likely to express these disorders as they would need two recessive alleles while males only need one because the Y chromosome does not carry a the same gene that can cover up the problem with a dominant allele.
Pedigrees tricks
Autosomal dominant:
- One of the parent has to have the allele
- Seen in every generation
Autosomal recessive:
- Skips generations
- A child has but none of the parents do
Sex linked
- Usually only males get it
- If a female gets it then her father must have it
Radiation and causes of mutation
- Chemical mutagens
- Radiation
Increase the mutation rates and can cause genetic diseases and cancer
Nuclear bombing of Hiroshima
- Over 100 000 people died around the time of the bombing from fires, the explosive shockwave, and Acute Radiation Syndrome (ARS).
- ARS occurs when a person is exposed to a large dose of ionising radiation.
- Lower doses cause nausea and headache, larger doses also cause seizures and fever, and very high doses cause death.
- Individuals who survived higher risk of certain types of cancer, especially leukemia, due to the increased number of mutations.
Accident at Chernobyl
- Released far more radioactivity than the atomic bomb used in Hiroshima
- failed test led to the meltdown of one out of four reactors
- A complete meltdown was averted by workers who remained in highly radioactive conditions to stabilise the plant, some of whom died of ARS.
Explain Mendel’s pea experiment
- First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
- Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
- These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)
Findings from results:
1. When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed. E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants
2. When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1. E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1
What happens to the two alleles of each gene during meiosis?
The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes. Which genders are more prone to dominant and recessive traits?
As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous.
Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease).
Human males have only one X chromosome (and therefore only one allele). X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele.
- Females can be a carrier of a disorder without expressing it (heterozygote for a recessive disease condition)
- Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
- Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)
Can radiation and mutagenic chemicals increase the mutation rate of cell DNA and cause genetic disorders and cancer?
Yes
