Mendelian Genetics Flashcards
Describe the overview of Mendels Pea plant experiment
Strict control over which plants mated with which
Each plant has both male (stamens) and female (pistil) reproductive organs
Medel looked at 7 characteristics of pea plants with contrasting traits
What is the difference between a characteristic and a trait
Characteristic - heritable feature (physical feature)
Trait - each variant of a character
Describe the first step of Mendel’s pea plant experiment
Mendel grew true breeding plants – plants that always produce offspring with the same traits
Flower Colour:
True-bred plants with purple flowers will always produce plants with purple flowers
True-bred white will only produce white.
Mendel called this the P generation (parent generation)
Describe step 2 of Mendel’s pea plant experiment
Mendel set up monohybrid crosses
True bred plant with purple flower crossed with true bred plant with white flowers
Produced F1 generation
All F1 generation plants had purple flowers
With blending, inheritance would expect plants to be intermediate in colour
Mendel hypothesised that when the purple factor was present the recessive factor (white colour) did not show
Mendel called the purple flower colour the dominant factor
What was step 3 of Mendel’s Pea plant experiment
F1 generation self fertilized
Plants with white flowers reappeared in the F2 generation
Mendel recorded 705 plants with purple flowers and 224 with white flowers
Approximately 3:1 ratio
Medel found similar findings with other traits
What did Medel conclude from his pea plant experiments
An organism inherits 2 factors (we now know as alleles) for a characteristic
- Hereditary Particles - Genes
- Alternative Forms - Alleles
When the plant reproduces these 2 characters separate (segregate during gamete formation and end up in different gametes)
What is Medel’s law of segregation
Each gamete (sex cell) gives one factor (allele)
Each offspring gets one factor (allele) from each parent
Random fusion during fertilisation
What is a dominant trait
The observed trait
What is the recessive trait
The masked trait
If the organism has two dominant alleles then
the dominant trait will be expressed (PP)
If the organism has one dominant and one recessive allele
the dominant trait will be expressed (Pp)
If the organism has 2 recessive alleles
the recessive trait will be expressed (pp)
From Mendel’s Pea plant experiment, which trait was dominant and which one was reccessive
Purple was the dominant allele
White was the recessive allele
What are alleles
Different versions of a gene
E.g. the purple flower allele and the white flower allele are different versions of a gene at the same location on homologous chromosomes
Mendel also conducted experiments on more than one trait, setting up dihybrid crosses
What did he observe
That one trait did not affect another trait
So different factors (genes) segregate independently from one another during the formation of gametes
Mendel’s law of independent assortment: genes get shuffled - these many combinations are one of the advantages of sexual reproduction
Example: Dihybrid cross: cross true-breeding yellow round pea with true breeding green wrinkled peas
What are some recessively inherited disorders
Cystic fibrosis, Tay Sachs disease, Sickle cell anemia
The disease only shows in homozygous recessive individuals - inherit recessive allele from both parents
Heterozygotes are phenotypically unaffected but act as carriers of this disorder
Vast majority of diseases are recessive
What are some examples of dominantly inherited disorders
Achondroplasia (dwarfism), Huntington’s disease
Most harmful alleles are recessive but some disorders due to dominant alleles
How many human chromosomes are there
How many pairs of human chromosomes are there
46 chromosomes - 23 pairs of homologous chromosomes (maternal set and a paternal set)
22 pairs of autosomes, 1 pair of sex chromosomes
What is a diploid cell
diploid number of chromosomes 2n=46
2 sets of chromosomes
What is a haploid cell
Haploid number of chromosomes n=23
1 set of chromosomes
What is mitosis
What is meiosis
Mitosis: process by which cells are replicated for growth, repair etc
Meiosis: process by which gametes are produced. Produces 4 daughter cells, each haploid (n) containing a single set of chromosomes
Meiosis 1 has reductional division, meaning what
homologous pairs are separated reducing chromosome number by half
Meiosis 2 has equational division, meaning what
sister chromatids are separated producing 4 haploid gametes
Independent assortment of chromosomes during meiosis leads to what?
an individual producing a collection of gametes that differ greatly in the combinations of chromosomes inherited from parents
For humans (n=23) the number of possible combinations of maternal and paternal chromosomes in the gametes is 2^23 = 8.4 million
Why is each chromosome not exclusively maternal or paternal origin
Because of the process of crossing over
What is crossing over
How often does it occur
When does it occur during meiosis
Produces recombinant chromosomes that carry genes derived from two parents. The DNA of 2 non-sister chromatids (maternal and paternal) are joined - piece of maternal chromatid is joined to paternal chromatid beyond the cross over point and vice vera
Creates new combinations of maternal and paternal alleles
Average of 1-3 crossing-over events per chromosome pair
Occurs in prophase 1 when each gene on one homolog is aligned precisely with that on the other homolog
What is the benefit of random fertilisation
Radom nature of fertilisation adds to genetic variation arising from meiosis
In humans each chromosome represents 1 of 8.4 million (2^23) possible chromosome combinations
2^23 * 2^23 = 70 trillion diploid combinations
Describe situations when offspring don’t match mendelian patterns of inheritance
Genotypic ratios follow Mendel’s laws but phenotypes do not
E.g degree of dominance
Alleles can show differing degrees of dominance and recessiveness in relation to each other
What is incomplete dominance
The phenotype of the heterozygote is intermediate between the phenotype of the dominant and recessive traits - blending of the traits
One trait is not really dominant over the other
Give examples of incomplete dominance
Snapdragons
Human hair - curly (homozygous) or straight (homozygous)
Heterozygous = wavy hair
Another example of non-mendelian patterns of inheritance is Codominance
What is this?
When both alleles for a trait are expressed in heterozygous offspring
The two dominant genes are expressed at the same time
Give an example of Codominance
Glycoprotein on the surface of red blood cells
2 forms: M and N
LmLm = M
LnLn= N
LmLn = MN type
ABO blood group
What does it mean by multiple alleles, give an example?
Most genes exist are more than two allels (not just two forms as in Mendel’s Pea plant) e.g. ABO blood group
3 allels of a single gene: Ia, Ib, Io
4 phenotypes: A, B, AB or O
What does Pleiotriopy
Most genes have multiple phenotypic effects
For example: Pleitropic alleles are responsible for multiple symptoms associated with hereditary diseases such as cystic fibrosis and sickle cell anemia
What is Epistasis
Phenotypic expression of gene at one locus affects another gene at a different locus e.g. labrador coat colour
Explain how epistasis affects coat colour in labradors
Black colour is dominant to brown
BB, Bb = black or bb = brown
Second gene determines whether pigment will be deposited in hair
Dominant allele E results in deposition of either black or brown pigment
If the dog is homozygous recessive for the second locus (ee) then no deposition = yellow coat
Means there is no longer a 9:3:3:1 ratio
What is polygenic inheritance
One characteristic influenced by many genes
Generally quantitative traits - continuous variation
Additive effect of two or more genes on a single phenotypic characteristic
E.g. height, skin colour
Skin colour is an example of polygenic inheritance
Explain how it works
At least 3 separately inherited genes
Simplified version: 3 genes ABC with a dark skin allele for each gene contributing one unit of darkness to the phenotype and being incompletely dominant to the other allele
* AABBCC - very dark
* aabbcc - very light
* AaBbCc - intermediate
As alleles have a cumulative effect AaBbCc would be the same as AABbcc
Give an example of when environmental factors will affect the phenotype for a character as well as the genotype
Hydrangea
Flowers of same genetic variety range in colour depending on soil pH
Name some instances where Mendel’s laws do not apply
- Mitocondria inheritance - mtDNA is solely inherited through the maternal line
- Linkage - two genes that are close together physically
- Linkage disequilibrium - two alleles that are not inherited separately
