Mendelian Genetics Flashcards
What did Mendel Study?
The principles of heredity
What’s are the two Hypotheses for heredity?
- The “blending” hypothesis is the idea that genetic
material from the two parents blends together to produce a trait. - The “particulate” hypothesis is the idea that parents pass genetic material as discrete heritable units that interact to produce a trait.
Why did Mendel use pea plants? (A,B,C)
Mendel used pea plants as a model organism to study heredity.
A. There are many varieties of these plants with distinct traits for various heritable characters.
B. Matings can be controlled.
C. Matings produce several offspring (from seeds)
Characters in Pea Plants
Mendel tracked characters that varied in an either-or manner such as purple or white flowers. Character = flower colour
Trait = Purple or White flower
True breeding plants
produce offspring of the same variety (traits) when they self-pollinate
Mendel’s Experimental Design
Parental Generation (P)
True-breeding (trait 1) x True-breeding (trait 2)
First filial generation offspring (F1)
Cross results in hybrids
First filial generation offspring (F1)
Second filial giration offspring (F2)
Mendel’s Three Principles
The principle of:
1. Segregation
2. Dominance
3. Independent Assortment
P generation
- true-breeding parents (ex: purple flowers x white flowers)
- When Mendel crossed true-breeding white and purple flowered pea plants, all of the F 1 hybrids were purple.
F1 generation
-hybrids
- When Mendel crossed the F1hybrids, many of
the F2 plants had purple flowers, but some had white.
F2 Generation
Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation.
Mendel’s observations + results
- Mendel observed the same pattern of inheritance in six other pea plant characters.
- His results support the particulate hypothesis of heredity.
- Mendel developed a model to explain the 3:1 inheritance pattern he observed in F 2 offspring.
Mendel’s Model (5 points)
- For each character, an organism inherits one heritable factor from each parent.
- Alternative versions of these heritable factors account for variations in inherited characters. These alternate versions are alleles.
- One allele is dominant while the other is recessive.
- These heritable factors segregate (separate) during gamete formation and are packaged into different gametes.
- Sex of the parent passing on the allele does not affect the inheritance pattern.
Punnett Square
A Punnett square is a matrix where the rows represent the possible gametes of one parent, the columns the possible gametes of another parent, and the boxes the possible combinations in offspring.
What does a Punnett square allow you to predict?
Can allow you to predict the probability of an offspring’s genotype and phenotype for a particular set of parents.
Genetic ratios
- Phenotype vs genotype
- Phenotype = Purple, purple, purple, white = 3:1 ration
- Genotype= PP (homozygous dominant), Pp (heterozygous), Pp (heterozygous), pp (homozygous recessive) = 1PP: 2Pp: 1pp
What makes an Allele Dominant?
- When the effects of an allele can be detected regardless of the alternative allele, then that allele is described as dominant.
- If the effect of an allele is masked in the heterozygous condition, then the allele is described as recessive.
Mendelian Genetics and Human Disorders
- Mendelian traits in humans can be disorders.
- These disorders are caused by one gene (monogenic disorders).
- The disorder can be inherited as a recessive or dominant trait depending on the allele involved in the disorder.
Albinism
A recessive disorder
Heterozygotes
are referred to as carriers when the disorder is recessive.
- They are not affected by the disease but they are carriers of the disorder allele.
Achondroplasia
- a type of dwarfism
- a dominant disorder
What about the inheritance of two characters?
- Mendel constructed his model and derived the law of segregation by following a single character in monohybrid crosses.
- Mendel identified his third law of inheritance by following two characters at the same time in dihybrid crosses.
Two hypotheses exist
- Traits are inherited as a package; alleles are linked and assort dependently into the same gamete
- Traits are not inherited as a package; alleles are not linked and are independently assorted into gametes.
Dependant assortment
Parental combinations only
Independent assortment
Parental but also new combinations
The two hypotheses…
(Dependant and independent assortment) predict different phenotypic ratios in the F2 offspring when F1 dihybrids self-pollinate
Mendel’s model makes sense! Now we know…
- Chromosomes are the vehicles of heredity.
- How meiosis works and shuffles alleles into unique combinations.
What does Mendel’s model explain
The laws of segregation and independent assortment.
1. Genes are the units of heredity.
2. What genes are, how they work, and how mutation can change them.
3. How alleles arise and that some can be dominant.
Chromosome are Vehicles of Heredity
All our genes contributing to a specific character can be found at specific locations (loci) within a specific chromosome.
For every chromosome what do we inherit?
We inherit a pair called homologous chromosomes and, therefore, pairs of each gene.
These pairs of genes can differ at…
can differ at their nucleotide sequence and encode different versions of proteins (different alleles) or they can be identical and encode identical proteins (same alleles).
Different proteins
Can lead to different traits
The chromosome theory of inheritance
Was used to explain Mendel’s model in the early 1900s
Principle of segregation
The alleles for a seed color segregate during gametogenesis because they are located on homologous chromosomes and these chromosomes are separated during Meiosis I.
Principle of independant assortment
The alleles for seed shape and seed color assort independently because the genes are located on different chromosomes. During Meiosis I, maternal and paternal chromosomes from homologous sets are randomly distributed into daughter cells.
Genes are
Units of heredity
A. Genetic information flows from dna to rna to proteins
- Mice with this DNA sequence have dark coats
B. Differences in genotype may cause differences in phenotype (physical traits that are a product of the proteins produced)
- Mice with this DNA sequence have light coats
Explanation for a dominant Allele
- Many recessive alleles are alleles that have been changed through mutation (called mutant allele) and produce non-functional proteins.
- In complete dominance, having one copy of the normal allele produces enough functional protein to produce a normal phenotype.
Protein P
- functions as an enzyme
- the amount of functional protein P is the molecular connection between the genotype and the phenotype
- the normal (dominant) allele encodes a functional enzyme
Genotype is
The amount of functional protein P produced
Phenotype is
The relationship of the normal (dominant) and mutant (recessive) alleles display simple Mendelian inheritance
Physical traits that are a product of the proteins produced
Mendel’s model doesn’t explain it all
Not all characters are Mendelian characters! The inheritance pattern of MOST characters do not follow his simple model.
