3.4 Genes Flashcards
Chromosome
A bundle of DNA containing many genes
Alleles
Different versions of the same gene
Phenotype
Physical appearance
(blue eyes)
Genotype
The combination of alleles which you carry for a particular gene
Dominant allele
An allele which is always expressed when it is present
- Capital letter
It cannot be masked by another allele
BB or bB or Bb
Recessive allele
An allele which can be masked by another allele
- small letters
it is only expressed when two copies are present.
must be bb, no B
Homozygous
Individual has two copies of the same allele of a gene on a pair of chromosomes
(same)
Heterozygous
Individual has two different alleles of a gene on a pair of chromosomes
(other)
Diploid cells
A cell which contains two copies of each chromosome
haploid cells
A cell which contains one copy of each chromosome
production of sperm cells and ova occurs through…
meiosis
Inheritance of eye colour
Alleles of sperm and ova determine phenotype of baby
Same rule applies to inheritance of plants
If there was a dominant allele (B) eye color would show dominant (brown)
Dominant alleles are twice as likely to be shown…
Ratio of
RR : Rr : rr
1 : 2 : 1
25%, 50%, 25%
Ratio of
RR : Rr : rr
RR : Rr : rr = 1 : 2 : 1
RR : Rr : rr = 25%, 50%, 25%
in the genetic cross Rr x Rr, what is the ratio of phenotypes?
Dominant : Recessive = 3 : 1
Drawing a punnet sqaure
- Write down the genotypes of the parents (Rr x Rr)
- Write down the genotypes of the gametes that each parent produces. (R +r) & (R +r)
- Draw your Punnett Square
- Fill it in
This shows you the possible genotypes of the offspring
Must draw a key for what the Phenotypes are
Carrier
a heterozygous genotype which shows the dominant phenotype is a carrier for an allele which is not expressed in the phenotype
Aa
- Shows A, but is a carrier for a
Test cross
An experimental cross of an individual organism of dominant phenotype but unknown genotype to an organism with a homozygous recessive genotype
AA x aa → all normal pigment (Aa)
EG - photo
The male has one of two genotypes : BB or Bb. This male is crossed to a black-bodied female of genotype bb. There are two possible outcomes:
Mendels discovery
He crossed pea plants and dicovered the basic princeples of heredity with the idea of ‘dominant’ and ‘ressesive’ genes.
Disocvered:
- organisms possess two versions of each factor
- Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
Co-Dominance
2 diffrerent alleles show up in 1 phenotype
- both alleles are dominant and show in phenotype
Inheritance of ABO blood groups
A blood = Carbohydrate A attached
B blood = Carbohydtate B attached
O blood = Neither carbohydrate
Use Punnet square to solve which blood type baby may inherit from parents *
Inheritance of cystic fibrosis and Huntington’s disease
Cystic fibrosis:
- autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
- mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
- CAN be a carrier
Huntington’s Disease:
- autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
- Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia
- CANNOT be a carrier
Inheritance of Sickle-cell disease
Autosomal recessive
- Makes blood cells shaped like sickles
- Causes blood clots
sex-linked diseases
characteristic is located on a sex chromosome (X or Y)
Y chromosome much shorter - 78 genes
X chromsome longer - 2000 genes
Pattern of inheritance on sex linked chromsomes
As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous
- X-linked dominant traits are more common in females (twice chance to have one)
Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits
- X-linked recessive traits are more common in males (cannot be masked by another allele)
Red = disease
Red-green colour blindness and haemophilia
Haemophilia:
- recessive sex-linked trait
- body’s ability to control blood clotting is impaired
- fibrin formation is prevented due to defective coagulation factors on the X chromosome
Red-Green Colour Blindness:
- recessive sex-linked trait
- caused by a mutation to the red or green retinal photoreceptors on the X chromosome
- diagnosed using the Ishihara colour test (circles and colours showing a number)
Radiation and mutagenic chemicals
increase the mutation rate and can cause genetic diseases and cancer
- The nuclear bombing of Hiroshima and accident at Chernobyl are two examples of a catastrophic release of radioactive material
Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl
long-term consequences:
- increased incidence in cancer development
- Reduced T cell counts and altered immune functions, leading to higher rates of infection
- organ-specific health effects
Pedigree charts
graphic representations of inheritance in a family
2 affected must have affected off spring
Determining a recessive trait with pedigree charts
Skips a generation
Determining a Dominant trait with pedigree charts
Most likely dominant due to trait in all generations
Identifying autosomal dominant with pedegree charts
Identifying autosomal recessive with pedegree charts
If both parents are unaffected and an offspring is affected, the trait must be recessive
- Both male and females affected
Identifying Sex linked with pedegree charts
usually only male affected
- not possible to confirm sex linkage from pedigree charts, as autosomal traits could potentially generate the same results
