Mendelian genetics Flashcards
Describe the history of genetic modification:
- People have recognized for years that traits are inherited
- Used this to their advantage in “selective breeding”
- Purposely mated individual animals (and plants) to select for certain traits
- Eg. Dog breeds
Who is Thomas Morgan?
- 1933 - discovered the role of genetics on chromosomes in heredity
- Worked with fruit flies
- When crossing flies with red eyes he found some offspring has white eyes
- This caused him to investigate heredity further (up until this point he was doubtful of theories from Darwin and Mendel)
Describe Mendel’s experiments:
- Mendel wanted to test patterns in genetics by observing traits such as flower color
- He let pea plants self-pollinate for several generations to ensure they were pure bred
- Then artificially crossed one type of plant with another to observe results – created hybrids (not purebred)
Define monohybrid cross:
A cross involving only one trait
What are Mendel’s generations?
P - purebred parents generation
F1 - First generation of hybrids
F2 - second generation of hybrids
What happened with the F1 generation?
- Only one trait (of the two possible parent traits) was observed in the offspring
- Mendel decided that the trait observed must be dominant
- The trait not observed was recessive
- In this case one trait showed complete dominance over the other (the recessive trait will not show at all if the dominant gene is present)
What happened with the F2 generation?
- Recessive traits reappeared in some offspring
- About 1 in every 4 offspring showed recessive traits
Describe the law of segregation:
- “All individuals have two copies of each factor. These copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every factor.”
- Note: we now know Mendel’s “factors” were genes
- Different forms of each gene are now called alleles
- Not all genetic combinations are this simple
Define gene:
A genetic code that designates a specific trait eg. Flower color
Define allele:
Form of a gene eg. Purple vs. white flowers
Define genotype:
Combination of alleles for a trait
Define pheotype:
Physical trait observed (characteristic of an organism)
Define homozygous:
Two identical alleles
Define heterozygous:
Two different alleles (one is likely recessive, but not always)
Define dominant allele:
An allele that has the same effect on an organism whether present in homozygous or heterozygous state
Define recessive allele:
Only has an effect when present in homozygous state
Define carrier:
Has one copy of recessive allele that can cause genetic disease
Describe the relationship between gametes and alleles:
- Gametes are haploid and therefore only have one copy of gene (only one allele)
- When gametes fuse, diploid zygotes end up with two copies of a gene – may be the same allele or different alleles of that gene
How are genotypes symbolized?
- First letter for description of dominant allele is used
- Capital letter represents dominant allele
- Lower case letter (same letter) represents recessive allele
Describe punnett squares:
- Used to analyze/predict genetic crosses
- Used with F1 generation
Describe a test cross:
- Sometimes geneticists will do a test cross to determine the genotype of an individual
- Cross an individual with a dominant phenotype with an individual known to be homozygous recessive
- Used to check to see if dominant phenotype is a result of homo or heterozygous genotype
What is a dihybrid cross?
- Examination of two traits simultaneously
- Four genes (so two genotypes) are observed, instead of two
Describe Mendel’s law of independent assortment:
- The two alleles for one gene segregate (assort) independently of the alleles for other genes during gamete formation
- Translation: one gene does not generally affect the expression of another
How do you analyze a dihybrid cross?
- A punnett square can still be used to predict offspring but sixteen squares rather than four are necessary
- All possible combinations of the two different genes for females are written across the top, those for the male are written down the side
Describe co-dominant alleles:
- Sometimes both alleles can be considered dominant and both can be expressed
- End up with a mixture of traits
- Eg. Roan Animal: both black (B) and white (W) are expressed. (Individual black and white hairs). Genotype is BW
- Capital letters
Describe incomplete dominance alleles:
- Neither allele conceals the other
- End up with a mixed trait eg. Red and white combine to make pink
- Can be represented with subscript numbers or superscript letters:
- Eg. Red flowers: R1R1, Pink : R1R2, White: R2R2
- Eg. Sickle cell anemia, normal: HbAHbA, Some traits of sickle cell: HbAHbS, Full sickle: HbsHbs
Describe what happens when there are multiple alleles:
- Some genes have more than two alleles
- Each individual has only two alleles, but more than two exist within a population
- Many more patterns are possible than with just two alleles
Describe blood types in terms of genetics:
- There are three alleles for blood types (note, also separate gene for Rh factor)
- Type A and B are dominant (co-dominant) and O is recessive
- Alleles are IA, IB, and i which form A, B and O respectively, see chart on next slide
- Rh factor is a separate gene where Rh+ is dominant over Rh-
Describe sex-linked alleles:
- Some genes can be linked to one of the sex chromosomes (encoded for by the sex chromosomes)
- Most common are x-linked traits
Describe the inheritance of x-linked traits:
- If an x-linked trait is recessive, the phenotype is more likely to appear in males – why?
- The gene is not usually present at all on a Y chromosome so there is no chance of the dominant allele there
- Therefore a male only needs one recessive allele for the trait to appear
Describe color blindness:
- An x-linked recessive trait
- Much more common in males than female
- Possible genotypes and phenotypes for a female are:
- XNXN – Normal
- XNXn – Normal (but a carrier)
- XnXn – colorblind
- Possible genotypes and phenotypes for a male are:
- XNY – normal
- XnY – colorblind
- As you can see, there is nothing on the Y chromosome to “dominate” the recessive Xn trait if it does appear
How do we represent sex-linked traits?
- Females will be represented by 2 X chromosomes with superscripts to indicate the allele on that chromosome
- Males will be XY with the appropriate superscripts for the alleles
- Y does not usually have an allele attached to it, unless it is a y-linked trait
Describe Y-linked traits:
- Not as common as females cannot be carriers
- Because females don’t have Y chromosomes, men can only pass Y-linked genes to their sons
Describe barr bodies:
- Structure formed when the inactive X chromosome condenses tightly
- In females one X-chromosome inactivates and condenses into a Barr body in every cell
What happens in the genetics of calico cats?
- Gene for coat color in cats is located on the X-chromosome
- Calico cats occur when different X-chromosomes in different cells are de-activated into Barr bodies
- Eg. Cells that produce orange fur have the X chromosome containing “black” de-activated and vice versa
Describe lethal alleles:
- What if you get a “non-mendelian” ratio of offspring, such as 2:1
- Could be caused by a lethal allele
- Definition: a gene where an allele causes a lethal trait
- If the organism dies as an embryo, could get unexpected ratios of traits in offspring
- Death sometimes takes years
- Can be expressed as incomplete dominance when heterozygous
- Eg. Manx cats
Define chromosome mapping:
Process used to determine position of genes on chromosomes
Define map unit:
Distance between points where crossover is likely to occur
Define map distance:
Distance between genes on a single chromosome
How can we tell if a gene crossover has occured?
- Some genes are linked – encoded for on the same chromosome
- Linked genes do not assort independently, they assort together
- Therefore, linked genes in gametes should occur in the same phenotypic combinations as the parents
Describe an example of gene crossover:
- Two genes for sunflowers are encoded on the same chromosome – the gene for height and flower color.
- T = dominant, tall; t= recessive, short; Y = dominant, yellow; y= recessive, red
- You know that the Tall and Yellow genes are linked (because there is no record of tall red plants or short yellow plants in the pedigree)
- You cross a tall yellow plant with a short red plant and 5% of the offspring are tall and red or short and yellow
- The small number of unexpected results indicates crossover must have occurred
Describe recombinants:
- Offspring that have different combinations of genes on a chromosome than the parents
- Recombination frequency – percentage of times crossover occurred when P1 found gametes
- Recombination frequency = number of recombinant types x 100% divided by Total # offspring
How do you use the recombination frequency?
- 1% = one map unit
- Can examine several genes and compare the distances between the genes to determine the order they come in
- Our sample calculation allows us to speculate that the genes for color and height of a sunflower are 10 map units apart
- We examine a third gene, seed type, and find out that it is 3 map units from color and 7 from height
What is a pedigree?
- A flow chart used to symbolize ancestry
- Shows patterns of relationships and traits in a family over many generations
- A way to study genetics without actually performing artificial crosses
Describe the numbers used on a pedigree:
- Roman numerals are used to indicate the generation
- Arabic (regular) numerals are used to indicate the specific individuals in a generation
How do we use a pedigree to study patterns?
- Can use a pedigree to work backwards to follow a certain trait
- For example, if a child shows up with an autosomal recessive trait, you know both parents must be carriers and at least one of each of their parents must be a carrier
