Genetics Flashcards
What is a locus
The physical location of a gene on a chromosome
What is a an P; F1 and F2 generation
P = Parental generation
F1 = The first fillal generation; offspring of the P generation
F2 = The second fill generation; offfspring of F1 generation
What is a donimant and recessive allele
Dominant = An allele that is expressed if present in the genotype
Recessive = An allele whose expression is masked by a dominant alleleW
What is heterozygous; homozygous
Hetero = Posseing different alleles of one gene
Homo = Possesing identical alleles of one gene
Explain phenotype and genotype
Pheno = An observable characteristic
Geno = An individuals allele combination for a particular gene
Explaun true breeding; wild type and mutant
True breeding = Homozygous; self-fertilisation yields offspring identical to see for a given trait
Wild Type = The most common phenotype; genotype or allele in a population
Mutant = A phenotype; genotype or allele resulting from a mutation in a gene
Define inheritance
- Something that is or may be inherited; property passing at the owner’s death to the heir or those entitled to succeed; legacy
- The genetic characters transmitted from parent to offspring.
- Something, as a quality, characteristic, or other immaterial possession - an inheritance of family pride.
Explain Mendel
- Gregor Mendel, the son of a farmer and a brilliant mathematician was a monk in Austria
- He worked with the garden pea plant
(Pisum sativum) which is capable of both
self-fertilization and cross-fertilization
Why did Mendel use peas
- They are easy to grow,
- Develop quickly,
- Produce many offspring and
- It is easy to control which plants mate with which
- Monoecious (both male and female parts in one flower)
What parts make up the stamen and carpel
Stamen = anther and filament
Carpel = Stigma; Style; Ovules in ovary
Explain peas being yellow
All peas are yellow when one parent produces yellow seeds and the other parent produces green seeds
Explain using pollen for plant A to fertilise the egg of plant A
- Self fertilisation
- A true-breeding plant is one that, when self-fertilized, it only produces offspring with the same traits.
- True-breeding organisms have identical alleles for specified traits. The alleles for these type of organisms are homozygous
- True breeding = all green seed ; all yellow seeds and then some green and yellow seeds
Explaun using pollen for plant A to fertilise the egg of plant B
- Cross fertilisation
- All yellow seed and then some yellow and green seeds
Explain punnet squares
- Named after R.C. Punnett
- It is a diagram that uses the genotypes of a two parents to reveal the which allele combinations their offspring may inherit
- In the punnett square above we have a 50% chance that offspring who will be Tt or tall and 50% chance that offspring will be tt or short
Explain Mendels first experiment
- Dealt with single traits that have two expressions – one gene, two alleles
He set up all possible combinations of crosses - He noted that some plants, through true-breeding or self- fertilization, always produced offspring identical to the parent plant
- Have identical alleles - The crosses involving some traits, however, produced more variable offspring
- Sometimes some traits vanished in one generation, only to reappear in the next
- One trait seemed to obscure the other
Mendel called the masking (visible) trait dominant and the trait being masked (blocked) recessive
( dominant alleles appear to mask recessive alleles )
What is a monohybrid cross
- Mating between two individuals that are heterozygous for the same gene
- Track the inheritance of one gene for a particular trait
Explain punnett squares in more detail
Uses the genotype of the parents to reveal which allele combinations the offspring may inherit ( probabilities )
What is the ratio in the genotype if two homozygous individuals are crossed
1 homozygous dominant
2 Heterozygous plants
1 homozygous recessive
What is the ratio in the phenotype if two homozygous individuals are crossed
3 Heterozygous dominant
1 homozygous recessive
Explain meiosis explaining Mendels Law of Segregation
- Mendel used his data to conclude that genes occur in alternative versions or alleles
- He deduced his law of segregation - the two alleles of each gene are packaged into separate gametes
– that is they segregate or move apart from each other during gamete formation - He further determined that each individual inherits two alleles for each gene and that these alleles may be the same or different
Explain Meiosis 1 and Mendels Law of Segregation
- During meiosis I, homologous pairs of chromosomes separate
and move to opposite poles of the cell - A plant of genotype Yy therefore produces equal numbers of gametes carrying Y or y, whereas genotype YY, a plant produces only Y gametes
- When gametes from the two parents meet at fertilization, they combine at random
What is the law of probability
The probability of two or more independent events occurring together, is the product (multiplication) of their chances of occurring separately.
What traits did Mendel work with
- Stem length
- Pod shape
- Seed shape
- Seed colour
- Flower position
- Flower colour
- Pod colour
Explain dihybrid crosses fully
- A dihybrid cross is a mating between individuals that are
each heterozygous for two genes - Mendel also performed experiments where he followed
the inheritance pattern of two traits simultaneously, e.g.
– plants that differed in colour and height - In these dihybrid crosses each F1 offspring inherit two gene pairs each consisting of non-identical alleles
- Gives rise to 2 different traits
Explain genes on different chromosomes being inherited independently
- In the F2 generation, 4 different combinations were obtained due to the fact that the two characteristics behave independently of one another
- It does not matter with which allele for pod colour the gamete ends up with it will have a 50:50 chance of getting either of the alleles for height (refer to independent assortment in meiosis)
- Brings in Mendels Law of Independent Assortment
What is Mendels Law of Independent Assortment
During gamete formation, the segregation of the alleles for one gene does not influence the alleles for another gene
What was Mendels findings ( basic law of inheritance )
- Dominant alleles appear to mask recessive alleles
A dominant allele is always expressed if present, a recessive allele is blocked - Meiosis explains Mendel’s law of segregation
-States that two alleles of the same gene separate into different gametes. Each individual receives one allele of each gene from each parent - Meiosis explains Mendel’s law of Independent assortment
- The inheritance of one gene does not affect the inheritance of another gene on a different chromosome
Explain the exceptions to Mendel
- Linkage groups which affects crossing over
- The traits that Mendel studied were the results of genes located on different chromosomes
- These genes were independently assorted from one another
- However, we do find that chromosomes carry different genes for different traits
- It has been found that many genes located on a specific chromosome tend to end up together in the same gamete; they are called linkage groups (found on the same chromosome)
Explain the exceptions to Mendel in more detail
- Linked genes located on the same chromosome
- Linkage groups are inherited together as they occur on the same chromosome
The ratios for dihybrid crosses for linkage groups are different than that predicted by Mendel - Crossing over may separate alleles especially if alleles are far apart from each other
Explain linked genes
- Linked genes are carried on the same chromosome and they are therefore inherited together.
- Unlike genes on different chromosomes they do not assort independently during meiosis ( because they are on the same chromosome very close to each other they will be inherited into the same gamete together )
Explain T H Morgan
- T H Morgan (1910) and his co-workers worked with the fruit fly Drosophila melanogaster and they confirmed that each gene has a specific location on a chromosome.
- Their initial observations showed that the ratios for the dihybrid (two traits) crosses were not the same as Mendel’s laws (9:3:3:1)
Explan Morgan’s findings
- Genes for eye colour and wing shape for flies were on the same chromosome (X) and that they tend to be inherited together,
i.e. red eyes with normal wings, and white eyes with vestigial (underdeveloped) wings
- However, in some of his crosses he found progeny/offspring with red eyes and vestigial wings, and white eyes and normal wings
- This is the result of crossing over
- The further apart genes are located on a specific chromosome the greater the chances of crossing-over taking place
Explain crossing over affecting
May affect the way linkage groups are passed on from one generation to the next
What were Morgan and his students able to come up with
Using the outcomes from Morgan and his students experiments, they were able to come up with the idea of mapping genes on a chromosome
- Knowing exactly where a gene lies is valuable information
Explain gene expression altering Mendelian ratios
- Another exception to Mendels Law
- We do encounter situations where there is no such clear-cut dominance to be observed,
i.e. the offspring formed do not resemble either parent in appearance.
Explain the first exception to the dominance rules
- Incomplete dominance
- When true-breeding red snapdragons (flower plant) are crossed with true-breeding white ones the F1 plants are all pink-flowered
- To prevent confusion between complete dominance and incomplete dominance we write both alleles in the capital form, but we subscript or number the one set/homologue,
e.g. R1R1 for red, R2R2 for white. Pink will thus be R1R2
- The F2 generation will have three possible colours, i.e. red, pink and white.
- The F2 thus have three different phenotypes and three different genotypes ( ration = 1 : 2 : 1 )
Explain co-dominance
- In this case we find that no allele in a gene allele pair is completely dominant over the other one.
- However, unlike incomplete dominance where the heterozygote expresses a mixture of the two extremes, in codominance the heterozygote displays characteristics of both alleles equally.
- The best quoted example is in the case of blood groups in humans.
Explain an example of co-dominance
- Landsteiner (1900) discovered the multiple allele A-B-O blood group in humans. Any individual will, however, have only two of the alleles present
- Glycolipids on the membrane of the red blood cells determine whether donated blood is compatible with that of the recipient.
Why are the trait inheritance patterns difficult to see
- In the case of children looking more like grandparents than parent
- One gene may influence the phenotype in many ways
- Multiple genes may contribute to one phenotype
Explain Pleiotropy
- When pattern is hard to see
- A gene that affects more than one characteristic of an individual
- Has multiple effects on the body and therefore produces many different phenotypes
What are examples of pleiotropy
- Sickle cell
- Marfan syndrome
- Phenylketonuria
- Porphyria variegata
Explain sickle cell
- The allele responsible for sickle-cell anaemia can be found on the short arm of chromosome 11.
- A person who receives the defective gene from both father and mother develops the disease;
Explain Marfan syndrome
Abnormality in fibrillin production
Explain Phenylketonuria
- An inherited disorder that affects the level of phenylalanine in the body
- Phenylalanine -amino acid that can be obtained from food.
- Causes this amino acid to increase in amount in the body, which can be very dangerous.
- The human disease is caused by a defect in a single gene on chromosome 12 that affects multiple systems,
Explain Porphyria variegata
- Lack of liver enzyme that results in a build up of porphyrins
- Accumulates in the urine ( leads to dark coloured urine ) ; digestive system ( leads to constipation; abdominal pain ) ; muscles ( leads to rapid pulse; weak limbs ) ; nervous tissue ( leads to delirium; stupor; convulsions; mad behavior )
