Mendelian genetics Flashcards
Who was Gregory Mendel?
An Austrian Monk who conducted experiments on pea plants to investigate inheritance and proposed three principles:
1) Law of independent assortment
2) Law of dominance
3) Law of segregation
He demonstrated that characters are transmitted as particles of information between generation (genes)
What is blending inheritance?
Offspring are a dilution of different parental characteristics
Why were pea plants ideal to investigate?
He had strict control over which plants mated (self-fertilization is an option) and the environment
Each pea plan has both male (stamens) and female (pistill) reproductive organs
Mendel’s experiments
1) He grew true breeding purple and white plants (produce offspring with the same traits when self-fertilized) to create a parent generation
2) He created a set of monohybrid crosses (purple X white) to find the F1 generation and discovered all offspring had purple flowers (dominant trait)
3) F1 generation was self-fertilized and white flowers reappeared in the F2 generation in a ratio of 705 purple to 224 white (3:1)
What did Mendel conclude?
An organism inherits 2 factors (allele) for a characteristic and when the plant reproduces these two characters separate = law of segregation
Each gamete gives one allele to each offspring and each offspring gets one allele from each parent
What is the relationship between dominant and recessive traits?
Homozygous dominant = dominant trait is expressed
Heterozygous = dominant trait is expressed (recessive masked)
Homozygous recessive = recessive trait is expressed
What is the phenotype?
The physical description of the trait expressed
What is the genotype?
The genetic makeup of each trait (PP/Pp/pp)
Exploring the 3:1 ratio
One homozygous dominant : two heterozygotes : one homozygous recessive
1:2:1
What are alleles?
Different versions of the same gene with variation in the sequence of nucleotides bases at the specific gene locus
What did Mendel explore in his dihybrid crosses?
Observed that one trait did not affect another trait and identified that different genes segregate independently from one another during gamete formation.
Genes get shuffled and these combinations are one of the advantages of sexual reproduction
EXAMPLE: Mendel’s dihybrid cross
A true breeding yellow round pea plant was crossed with a true breeding green wrinkled pea plant
Observed that yellow is dominant to green and round is dominant to wrinkled
9:3:3:1 9 = yellow/round peas 3 = green/round peas 3 = yellow/wrinkled peas 1 = green/wrinkled peas
What are inherited recessive disorders?
Only show in homozygous recessive individuals, where both copies of the recessive allele must be inherited
Heterozygotes are phenotypically normal but carry the disorder
CYSTIC FIBROSIS
What are inherited dominant disorders?
Only require one dominant allele to be present = no carriers
Achondroplasia
What is the genetic makeup of humans?
46 chromosomes
23 pairs of homologous chromosomes (one maternal and one paternal)
22 pairs of autosomes and 1 pair of sex chromosomes
Diploid and Haploid Definitions
Diploid - 2n = 46 (2 sets of chromosomes)
Haploid - n = 23 (a set of chromosome)
What is mitosis?
Cells are replicated for growth and repair (sometimes asexual) to produce two daughter cells = diploid
What is meiosis?
Gametes produced/four daughter cells = haploid
Meiosis I = reductional division in which homologous pairs are separated - reduces chromosome number by half
What is meiosis?
Gametes produced/four daughter cells = haploid
Meiosis I = reductional division in which homologous pairs are separated - reduces chromosome number by half
Meiosis II = equational division in which sister chromatids are separated producing four haploid gametes
What is independent assortment of homologous chromosomes?
Produces a collection of gametes that differ in the combinations of the chromosomes inherited from the parents
What is crossing over?
Produces recombinant chromosome that carries genes derived from two different parents.
In Prophase I, DNA of two non-sister chromatids are joined and a piece of maternal and paternal chromatids is crossed over beyond the cross over point
What is random fertilization?
Adds to the genetic variation
Why do not all traits follow Mendel’s laws?
Genotypic ratios follow Mendel’s laws, but phenotypes do not
Mendel’s laws do not apply
What is codominance?
When both alleles for a trait are expressed in heterozygous offspring
What are multiple alleles?
Most genes exist as more than two alleles like the ABO blood group which results in four phenotypes:
A (AA/AO)
B (BB/BO)
AB = codominant = both glycoproteins present
O (OO) = recessive
What is pleiotropy?
Most genes have multiple phenotypic effects and are responsible for the multiple associated with hereditary disease
Cystic fibrosis and sickle cell anaemia
What is epistasis?
When the phenotypic expression of a gene at one locus affects another gene at a different locus.
The effect of one gene is masked by the effect of a gene at a different locus and the hidden gene is hypostatic.
Epistatic interactions occur when the genes involved encode enzymes in the same biochemical pathway
What is polygenic inheritance?
When one character is influenced by many genes = the additive effect of two or more genes on a single phenotypic character
Opposite of pleiotropy
What are the instances when the phenotype for a character depends on the environment?
Mitochondrial inheritance - mtDNA is solely inherited through the maternal line
Linkage - two genes close together on the chromosome
Linkage disequilbirum - two alleles that are not inherited separately
What is haploinsufficiency?
When one normal allele is not enough to result in a normal phenotype in the heterozygous individual - the mutant phenotype is dominant and can lead to an intermediate phenotype.
The presence of half the normal amount of a protein (from one functional allele) is not enough to give the normal phenotype
What is haplosufficiency?
When one normal allele is enough to result in a normal phenotype in a heterozygous individual. The mutant phenotype is recessive and heterozygotes are phenotypically normal but are carriers of disease
When do gene interactions occur?
When a new phenotype is produced dependent on the action of alleles of several genes
Gene 1 = RR
Rr = red and rr = yellow
Gene 2 = CC
Cc = no chlorophyll cc = chlorophyll
9:3:3:1
Red:brown:yellow:green
R-C-:R-cc:rrC-:rrcc
What is penetrance?
The probability that an individual with an appropriate genotype will show a change in phenotype
What is expressivity?
The degree of phenotypic change produced by a genotype which can be variable
What are genome maps?
Show the positions of the genes on the chromosomes
Based on recombination and linkage
What are physical maps?
Show the complete DNA sequence of a genome with positions of genes mapped
What is linkage?
Alleles of genes that are close together on the same chromosomes and tend to be passed on together to offspring
What is recombination?
When the chromosomes pair during meiosis to form gametes and pieces of DNA get swapped between them which mixes up the combinations of alleles
The amount depends on the distance apart
