5.4.2 Heredity Flashcards
Gregor Mendel heredity (Year 10 recap)
- Austrian monk who experimented with the selective breeding of peas
- Found that inheritance does not occur according to the blending model in which traits from each parents are averaged together
- Found instead that most characteristics are passed directly from one generation to another, but some skipped several generations or at a ratio of 1:1 or 1:3 (one out of 3 plants were green or yellow)
- Came up with the idea of recessive or dominant alleles
Mendel’s pea experiment
- Crossbred two purebred peas (one green, one yellow)
- The offspring produced were all yellow. This was the F1 generation
In the F2 generation produced by interbreeding the F1 members, one offspring out of 4 was green - Concluded that yellow is dominant, the green was a recessive gene.
Heterozygous vs homozygous
- If the alleles are both the same, it is homozygous. If both alleles are dominant, it is homozygous dominant. If both are recessive, it is homozygous recessive.
- If the alleles for the same gene are different, it is heterozygous.
Mendel’s law
- Genes in homologous pairs can be for an alternative form of the same trait. E.g. the gene for hair colour might be blond or brown, the gene for hair style might be curly or straight.
- Polygenic inheritance: a trait whose expression is influenced by multiple alleles, not just one. E.g. eye colour is not determined by the dominance of a single allele, e.g. B over b, but influenced by many other factors.
Mendelian ratios
- Ratio of the likelihood of particular traits being expressed in his F1 and F2 peas based on the traits of the parent plants, P
- Only applies to traits that are controlled by a single gene, called monohybrid crosses
Dihybrid cross
WILL NOT BE IN THE TEST
- When 2 traits on different chromosomes are observed over generations
- Ratio for F2 is 9:3:3:1 (Dominant, mixed, mixed, recessive)
Linked genes
WILL NOT BE IN THE TEST
- Some genes do not follow Mendelian ratios and appears to break the Law of Independent Assortment
- These are likely to be found on the same chromosome linked genes
- Linkage group: a set of genes at different loci on the same gene that except for crossing over, tend to be inherited together
Morgan experiment
White eyes were expressed only with male flies.
The gene for eye colour located on the X sex chromosomes (only male XY)
The discover of genes located on allosomes but not directly related to sex is sex linkage.
Allosomes
sex chromosomes
Sex linkage in humans eg
X: Red-green colour blindness and haemophilia
Y: facial hair
Sex linkage is a type of gene linkage - in which there is different chance of phenotype expression in female v. male.
Non-Mendellian Ratios
Incomplete dominance:
- Dominant allele is not showing its complete dominance. Neither allele is dominant, and both are partially expressed. Present in heterozygous offspring.
- Ratio 1:2:1 (homozygous dominant:heterozygous mixed:homozygous recessive)
e.g. snapdragon plants can express pink flowers, andalusian fowl (black+white=bluish-gray)
Codominance:
- Both alleles express themselves to produce a “pathwork” phenotype.
Polygenic Inheritance:
- For continuous traits, in which a range of expressions controlled by several genes or alleles. A trait controlled by two or more genes (polygenes) contributing to the phenotype.
- Previously we have been looking at traits that are easily deteced as being discontinuous (present or not) that are the expression of a single allele.
- Height, eye colour, skin tone.
Multiple allele trait
two examples
Normally a gene has two alleles - however, in this case one gene has multiple alleles.
E.g. blood type in which the alleles are codominant (expressed equally)
Rabbit coat colour: C black, c(ch) chinchilla, c(h) himalayan, c albino - and C is dominant
- c(ch) shows incomplete dominance when crossed with c(h) or c
- c(h) shows dominance over c
- This is because c(h) encodes a defective enzyme, and does not work at higher temperatures. Therefore, the rabbit’s extremities are cooler, the enzyme functions there and makes pigment. It is not because it has an albino gene, but due to environment impacting gene expression
When answering this question, use: it is stopping/supressing/reducing expression, not dominance/recessive/co/incomplete dominance as that information is not available even if assumptions could be made.
Codominance
Two alleles are expressed equally in the heterozygous phenotype.
e.g. camellia flower (pink and white), roan coat colour, andalusian fowl
Variations involving single genes
- Multiple alleles: multiple alleles for a given gene (blood type)
- Incomplete dominance: two alleles produce an intermediate phenotype
- Codominance: two alleles are simultaneously expressed
- Pleiotropy: some genes affect many different characteristics, not hust a single characeristic
- Lethal alleles: some genes have alleles that prevent survival when homozygous or heterozygous
- Sex linkage: genes carried on sex chromosomes, showing different inheritance parrerns than genes on autosomal
Pleiotropy
One gene affects multiple characteristics
- If there is a mutation in this gene, there will be several impacts across charcteristics.
E.g. Marfan syndrome: unusually tall, thin fingers and toes, dislocations of the lens of the eye and heart problems. - Mutation of a single gene that encodes protein in making elastical fibrils that give strenghth and flexibility to the body’s connective tissues
reduce the amount of functional protein makde by the body, resulting in fewer fibril (heart)
fibrils act as storage shelves for growth factors, when there are fewer growth factors cannot be shelved and thus cause excess growth
Lethality
Many genes in an organism’s genome are needed for survival
If an allele makes one of these genes nonfucntional, or causes it to take an abnormal, harmful activity, it may be impossible to get a living organism as homozygous or even heterozygous
Polygenic inheritance
Continuous characteristics such as height, hair colour, eye colour, are controlled by multiple genes, each making a small contribution to the overall outcome
There are 400 genes for height
Pedigree - simple definition
Generation chart of genetic traits that are inherited in a family
Reading pedigree
Circle: female
Square: male
Read like a family tree
Disorders are shaded
Generations numbered in Roman Numerals from I
Pedigree analysis
Autosomal or sex linked trait: autosomal affects males and females equally. If you observe roughlyequal distribution of the trait across genders, the trait is likely autosomal
Sex linked traits
- X-linked recessive: males are often more affected, as they only have one X chromosome
- X-linked dominant: females are more often affected as they have two X chromosomes
- Y-linked trait: only males, passed directly from father to son
2: determine if (autosomal) is dominant/recessive
- Dominant: in every generation. Affected individuals have at least one affected parent, unaffected individuals cannot pass on the trait.
- E.g. Huntington’s disease
- Recessive: may skip generations. Affected individuals can have unaffected carrier parents
- E.g. cystic fibrosis
Autosomal domiant
- Traits appears in every generation through vertical transmission
- Male and female equally affected
- An affected individual typically has one affected parent
- If one parent is Aa and the other is unaffected aa –> 50% chance of passsing the trait
Autosomal recessive
- Trait may skip generations
- Male and female equally affected
- Parents of affected individuals are usually carriers (Aa Aa). If both are carriers: 25% aa 50% Aa 25% AA
- E.g. sickle cell disease
X-Linked Dominant
- Males and females can be affected, but females are more commonly affected due to two X chromosomes
- Affected males pass the trait to all their daughters (not their sons)
- Affected females have 50% chance of passing the trait to each child
- e.g. rett syndrome
X-Linked Recessive
Males are more commonly affected (they have only one X chromosone - females have two so a dominant X chromosome can prevent expression of disorder)
* Affected males inherit from carrier females
* Sons of carrier mothers have a 50% chance of being affected.
* No male to male transmission - if a male has the recessive X chromosome, they will express it.
* E.g.
Steps to analysis pedigree
- Identify patter of transmission
- Check for gender bias - autosomal or linked
- Look for skipped generations - skipped generatioins suggest recessive inheritance. Continued transmission suggests dominant inheritance
- Asess parent to child relationships: father to son tranmission - autosomal or Y linked. No father to son tranmission - X linked
If parents are not affected but they have affected children, or the trait skips generations
Recessive autosomal
If the mother is affected and the father isn’t, and only sons are affected…
Recessive sex-linked
Also all daughters will be carriers
If the father is affected, the mother isn’t a carrier and no children are affected.
X linked recessive
* Sons inherit mother’s unaffected chromosome
* Daughter’s inherit both mother and father’s X - and if they don’t express the trait, the mother’s unaffected chromosome must be dominant.
* Therefore the allele for the trait is recessive.
If the mother is affected, the father isn’t, 50% of sons and 50% of daughters will be affected
X-linked dominant
Test cross - dominant trait
To determine if an affected individual is heterozygous or homozygous dominant when the trait is known to be dominant
AA x aa = 100% affected
Aa X aa = 50% affected
Test cross - autosomal v. sex linked
Cross an individual expressing the dominant phenotype with a recessive individual.
If autosomal- same for males and females E.g. 1:1 (Aa x aa) or 100% expression (AA x aa)
If sex linked - gender spilit
Males will directly express the X-linked traits inherited from their mothers.
Females will follow a more typical dominant/recessive pattern.
