Genetics Flashcards
Genes
DNA units in chromosomes coding for a particular protein or characteristic
Allele
- A form of a particular gene
- Alleles occupy the same locus of a particular chromosome
- One gene/allele is inherited from each parent
Artificially crossed
Breeding controlled by humans, usually to create organisms with specific traits.
Dominant allele
An allele which is expressed over a recessive allele
Recessive allele
An allele not expressed in the presence of a dominant allele
Genotype
The genetic makeup of a particular organism
Phenotype
- The observable traits,
- determined by both genotype and the organisms environment
Locus
Specific place or position (on a chromosome)
Offspring
Children or young of particular parent(s)
Selective breeding
Intentional mating of two organisms to promote specific trait(s)
Self pollinate
Transfer of pollen from anther to stigma of same flower or plant
Pure/true-breeding
Self fertilized or two pure-breeding organisms consistently produce offspring with the same particular trait
Trait
- In genetics, a characteristic of an organism.
- Some are visible e.g. Eye colour
- Some not easily seen e.g. Blood type
Who was Gregor Mendel
- Austrian monk in mid 1800’s
- Quantitatively showed that parent plants pass on factors of inheritance (genes)
- some factors not expressed but still able to be passed on
Mendel’s focus group and what he did with them
- Pea plants
- Thousands of selective breeding events e.g. forcing self and cross-pollination examining particular traits
- Starting with true-breeding plants for a particular trait he crossed them with other true-breeding plants
- Recorded traits of offspring
Why pea plants for Mendel’s experiments?
- grow and reproduce quickly
- many traits in one species
- easy to control reproduction (naturally self-fertilizing but can be cross-fertilsed)
- True-breeding strains for particular traits already available to him
Seven traits Mendel studied in peas
- Seed shape
- seed colour
- flower colour
- pod shape
- pod colour
- flower position
- plant length
Mendel’s three laws
- Segregation
- Dominance
- Independent assortment
Law of segregation
- If there are two alleles controlling a trait, these alleles separate and go to different gametes when parent reproduces.
- During meiosis alleles randomly divided between different gametes
- Thus 2x2 possible combinations of the same alleles in successive generations are possible
Law of dominance
If an organism has different alleles, the trait of one of these will be visible (domnant) whole the other will be hidden (recessive)
Symbols for dominant/recessive alleles
- Capital letter for dominant and lower case letter for recessive.
- Letters with a different form of capital vs lower case are typically used
- e.g. Dd or Tt (instead of e.g. Pp or Oo)
Homozygous individual
An individual with only one kind of allele for a particular gene
Heterozygous individual
An individual with two kinds of allele for a particular gene
Law of independent assortment
Different traits are passed on independently of one another so that combinations of traits in parents do not always match combinations of traits in offspring.
Two cats both have brown fur. They have the following alleles: Bb and BB. Which is the same - their genotype or phenotype?
Phenotype
Two cats both have brown fur. They have the following alleles: Bb and BB. Which one is homozygous?
- BB as it has the same alleles.
- Bb is heterozygous.
Monohybrid cross
- Genetic cross between two individuals
- Each of whom has different alleles for a single trait
- And only two different possible alleles
- can be heterozygous or homozygous individuals
F1 generation
- First filial
- Filial means the relation of a child to parent
Incomplete dominance
- When alleles show incomplete or partial dominance
- Neither characteristic is dominant over the other
- Offspring can show a new, mixed characteristic when heterozygous
- E.g. White flower crossed with a red flower -> pink flower
Co-dominance
- Co-existing characteristics because of equally dominant alleles
- Both are present in the phenotype when heterozygous
- E.g. Black cow crosses with a white cow -> spotted black and white cow
Blood groups - type of dominance
Co-dominance
Possible blood types in humans
- A
- B
- AB
- O
Rhesus blood factor
- Rhesus blood antigen
- Two alleles Rh+ and Rh-
- Rh+ is dominant and expresses a cell surface antigen
- Rh- is recessive and does not express an antigen
Polygenic inheritance
- Traits that are determined by more than one gene
- Results in continuous variation in a population
- E.g. Hair colour, eye colour, intelligence
Dihybrid crosses
- Examining inheritance of two different traits at the same time
- E.g. Flower colour AND seed shape
Pattern of inheritance with two true breeding dihybrid individuals
9:3:3:1
Pedigree diagram
- Diagrams showing family relationships
- Older generations are at the top
- Symbols of a square = males and circles = females
- Horizontal lines directly connecting individuals show breeding
- Vertical lines with horizontal connectors show offspring
- A key (shading) indicates the known phenotypes
Pedigree diagram uses
- To determine the type of inheritance
- Evaluate the inheritance of genetic conditions in a particular family
- By genetic councilors to advise families with genetic diseases
Sex chromosomes
- The sex of an individual is determined by the presence of X and Y chromosomes.
- Also called gonosomes.
- XX = female XY = male
Chance of male vs female offspring
- Determined by the sperm
- 50% that it contains a Y-chromosome = XY male individual
- 50% that it contains an X-chromosome = XX female individual
Autosomes
Chromosome pairs 1-22 not involved in determining the sex of an individual
Sex-linked genes
- Genes carried on the sex chromosomes
- X-chromosome has over 1000 genes
- Y-chromosome only 78 genes
Male risk of sex-linked disorders
- Since male have only one X-chromosome they have a higher risk of a genetic disorder found on the X-chromosome.
- Even if that disorder is recessive for females, since males have only one X-chromosome, they are still affected.
Examples of sex-linked disorders
- Muscular dystrophy
- Haemophilia
- Cleft palate syndrome
- Colour blindness
Haemophilia
- X-chromosome linked
- Recessive for females Lack of a gene for blood clotting
- Affected individual (male with this faulty gene) bleeds excessively
- Females homozygous for this faulty gene usually aborted before birth.
Muscular dystrophy
- A degenerative disease most common in boys.
- X-chromosome linked recessive inheritance pattern.
- Progressive loss of muscle function.
Genetic counselling
Advised if:
- Have a child or family history of a genetic condition, chromosome abnormality of birth defect
- You or family have a history of miscarriages or stillbirths They help you understand risks of passing inherited conditions on to your children
