Biology Unit 4.3 - Inheritence Flashcards
Explain variability and genetics…
Gregor Mendel suggested that an organism’s characteristics are determined by units (genes), which could be passed from generation to generation (on chromosomes)
What are genes?
Basic units of inheritence compaosed of the chemical DNA, which can seperate, pair up and mutate, and is part of a chromosome which codes for a specific polypeptide
What is a locus?
Position of a gene on a chromosome, where a diploid have two and each matching pair is called a homologue
What is an allele?
Alternative form of the same gene, which is found at the same locus
Explain the different allele combinations?
- Heterozygous - different alleles
- Homozygous dominant - two dominant alleles
- Homozygous recessive - two recessive alleles
What is a dominant allele?
Allele that is expressed in the phenotype whenever it is present
What is a recessive allele?
Allele which is hidden when a dominant allele is present
What is a genotype?
The genetic makeup of an individual
What is a phenotype?
Description of how the genotype is shown in the organism
What is F1 and F2?
Shorthand to indicate the 1st and 2nd generation
What is Mendel’s first law?
Characteristics of an organism are determined by internal factors which occur in pairs, where only one of a pair of such factors can be represented in a single gamete
What is a monohybrid cross?
Simple cross, describing the inheritence of a single pair of genes, which can be demonstrated using a Punnet square
Ratio of dominant to recessive is 3:1
What is Mendel’s second law?
Each member of an allele pair may combine randomly with either of another pair
What is a dihybrid cross?
Describes the inheritence of two characteristics; two pairs of genes
Ratio of homozygous dominant:heterozygous:heterozygous:homozygous recessive is 9:3:3:1
How do you calculate the expected value is a Chi-squared test?
- Calculate the total number
- Multiply the total by the ration number
- Divide by the total number of possible genotypes
What is a null hypothesis?
States that there is no significant difference between the observed and expected results
Rejected if the Chi-squared number is higher than the critical value - significant difference between observed and expected results
Accpected if the Chi-squared number is lower than the critic value - no significant difference between observed and expected results
How do you calculate the degrees of freedom?
Number of classes - 1 = Degrees of freedom
What is a test cross?
Determines the true genotype which is achieved by cross dominant phenotypes with homozygous recessive plants
50% of offspring will show a recessive phenotype, meaning the unknown genotype must be heterozygous
What is co-dominance?
When both alleles are expressed in the phenotype
Explain sex chromosomes…
Non-autosomal chromosomes
Females - XX
Males - XY
What is sex-linked inheritence?
Alleles are carried on the X chromosome, where any recessive allele carried on the X chromosome wille be expressed in the phenotype in males as there are no pairs, thus no dominant alleles exist
What is haemophilia?
Possibly fatal sex-linked condition, which means the person cannot produce a blood clotting hormone, meaning they may bleed to death
Mainly affects males as allele’s locus is on the X chromosome meaning only one recessive allele is needed for the condition to be expressed
What is linkage?
When two different genes are found on the same chromosome and are inherited together, and passed into gametes together
Genetic corsses do not follow the typical Mendelian pattern
What is complete linkage?
Occurs whn the genes are close together on the same chromosome, where recombination due to crossing over is very unoikely, meaning the genes are inherited together
What is incomplete linkage?
- May occur when genes are far or further apart
- Crossing over at chiasmata occurs during prophase I of meiosis, which seperates linked genes
- Four types of gametes are formed
- Large number of parental types and a small number of recombinant types
- Equal numbers of parental types
- Recombinant types are found in equal numbers
- Further apart the genes the more likley they are to be seperated by crossing over
What are mutations?
- Any change in the size, order or structure of DNA in an organism, which can affect genes or entire chromosomes
- Those which occur during the formation of gametes are the only ones that can be inherited
- Most occur during crossing over in prophase I and non-disjuction in anaphase I and anaphase II
- Happen at random
- Affect natural selection and evolution
What is mutation rate?
Very low in organisms such as humans, but is far higher in organisms with a short life cycle and a high rate of meiosis
What environmental factors affect mutation rate?
- X-rays
- Gamma radiation
- Ultraviolet light
- Chemical such as polycyclic hydrocarbons in cigarette smoke
What are carcinogens?
Mutagens which increase the rate of mutation, where an increase in exposure to mutagens will increase the mutation rate
Summarise the effect of carcinogens on genes?
- Carcinogen is an agent that causes cancer and is described as carcinogenic
- Some mutagens are carcinogenic and cause mutations in DNA
- Tumour suppressor genes halt cell division when enough cell have been produced by regulating mitosis
- Mutation may prevent tumour suppressor genes from functioning
- Mitosis continues repeatedly and continuously forming a tumour, which may be harmless or benign
- If it spreads around the body it becomes cancerous
- Proto-oncogenes can be switched on continuously by mutations, meaning excessive proteins are produced causing repeated mitosis
- Mutated proto-oncogenes are called oncogenes, which cause cancer
- Happens in mutation rearranges the DNA in a chromosome, and places the proto-oncogene next to a DNA sequence that activates it permanently or if there is an extra copy of a proto-oncogene
- Tobacco contains 4000 chemicals, where over 40 are carcinogenic and 400 are known to be toxic
How can gene and chromosomal mutations occur?
- DNA can be copied incorrectly before cell division
- Chromosomes can be damaged, broken or fail to seprate
What is a gene mutation?
Caused by any change in the sequence of nucleotides in the DNA molecule, causing the polypeptide to have the wrong sequence of amino acids meaning it won’t function as it should
What are the different types of gene mutations?
- Duplication
- Insertion
- Deletion
- Inversion
- Substitution of bases
What is duplication?
Sequence of bases is repeated
What is insertion?
Organic base/extra nucleotide is added into a sequence
What is deletion?
Removal of an organic base from a sequence
What is inversion?
Two organic bases are swapped in the sequence
What is substitution of bases?
Organic base is replaced by another base
Summarise sickle-cell anaemia…
- Normal haemoglobin HbA is produced by allele HbA
- Mutant haemoglobin HbS is produced by allele HbS
- Mutation is a substitution in a gene coding for the beta haemoglobin polypeptide
- One base is susbtituted with another
- Triplet CTC codes for glutamate
- T is substituted for A, where CAC codes for valine
- This change affects the folding of the polypeptide and its properties
- Glutamate is polar, while valine is non-polar
- Low partial pressure the haemoglobin aggregates and precipitates
- Cell membrane collapses on the haemoglobin and becomes sickle shaped, and may break in the capillaries
- Symptoms include joint pain and organ damage
- Ability to carry oxygen in the blood is reduced, resulting in anemia and possible death
What are chromosomal mutations?
Mutations caused by changes in the structure or number of chromosomes
How does the chromosome structure change?
Usually occurs during meiosis, where in prophase I, homologous chromosomes pair up and mistakes occur during crossing over at chiasmata, meaning pairs may end with different gene sequences
How does the number of chromosomes change?
Duaghter cells end up with the wrong number of chromosomes as paired chromosomes line up on the crowded equator, where an error occurs during anaphase, meaning chromosomes are not equally shared
Summaise Down’s syndrome…
Disjunction:
* Mutation affects chromosome 21
* Disjunction happens during oogenesis/meiosis at anaphase I and II
* May be due to faulty spindle fibres
* Chromosomes are not shared equally between daughter cells
* Secondary oocyte either has no chromosome 21 or has two copies
* First is unviable, while the latter will form a zygote with three copies of chromosome 21
* Individual will have 47 chromosomes
Translocation:
* Fragment of one chromosome attaches to another
* Fragment of chromosome 21 attaches to chromosome 14
* Abnoraml secondary oocyte fuses with a normal sperm, producing a zygote with two normal copies of chromosome 21 and an additional one attached to chromosome 14
* 5% of people with Down’s syndrome have 46 chromosomes
What is the control of gene expression?
DNA can be modified post-replication, which changes the ability of a gene to be transcribed and can be caused by the environment
What are epigenetic modifications by DNA methylation?
Cytosine can have a methyl or hydroxymethyl group added, where heavily methylated regions may not be recognised and are less likely to be transcribed
What is histone modification?
Histone proteins can be modified, where unmodified histones allow DNA to be packed more tightly, making DNA less accessible to enzymes, reducing transpiration, but when they are modified DNA is coiled more loosely allowing transcription enzymes to access particular sections of DNA
What is DNA damage?
Process which occurs about 60,000 times a day, where most is repaired, but epigenetic changes remain at the site of repair
What are the consequences of epigenetic changes?
- Genomic imprinting - gene may be permanently switched off by DNA methylation, which may lead to a medical condition
- X inactivation - whole chromosome is switched off, where in male mammals the other chromosome is inactivated and becomes a mass of densely staining chromatin called the Barr body
- Implicated in autoimmune decisions, mental illness, diabetes, and many cancers
