Chapter 14 - Mendelian Genetics Flashcards
Co
= together
Di
= two
Geno
= offspring
Hetero
= different
Homo
= same
Mono
= one
Pedi
= a child
Pheno
= appear
Pleio
= more
Poly
= many
Gene
= produce
Alleles
Alternative versions of a gene that produce distinguishable phenotypic effects.
Carrier
An individual who is heterozygous at a given genetic locus, with one normal allele and one potentially harmful recessive allele.
Codominance
The situation in which the phenotypes of both alleles are exhibited in the heterozygote.
Complete Dominance
The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.
Dihybrid
An organism that is heterozygous with respect to two genes of interest.
Dominant Allele
An allele that is fully expressed in the phenotype of a heterozygote.
Epistasis
A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited.
F1 Generation
The first filial, or hybrid, offspring in a series of genetic crosses.
F2 Generation
Offspring resulting from interbreeding of the hybrid F1 generation.
Genotype
The genetic makeup, or set of alleles, of an organism.
Heterozygous
Having two different alleles for a given gene.
Homozygous
having two identical alleles for a given gene.
Hybridization
The mating, or crossing, of two true-breeding varieties.
Incomplete Dominance
The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.
Law of Independent Assortment
Mendel’s second law
Stating that each pair of elleles segregates independently during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes.
Law of Segregation
Mendels first law.
Stating that each allele in a pair separates into a different gamete during gamete formation.
Monohybrid
An organism that is heterozygous with respect to a single gene of interest.
P Generation
The parent individuals from which offspring are derived in studies of inheritance.
P stands for parental.
Phenotype
The physical and physiological traits of an organism that are determined by its genetic makeup.
Pleiotropy
The ability of a single gene to have multiple effects.
Polygenic Inheritance
An additive effect of two or more gene loci on a single phenotypic character.
Punnett Square
A diagram used in the study of inheritance to show the results of random fertilization in genetic crosses.
Recessive Allele
An allele whose phenotypic effect is not observed in a heterozygote.
Testcross
Breeding of an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype.
Trait
Any detectable variation in a genetic character.
True-breeding
Referring to organism that produce offspring of the same variety when they mated with the same varieties; are homozygous for characters being considered.
Modern Genetic Theory
Based on particular inheritance in which heredity is passed from parent to offspring by discrete units called genes.
Gregor Mendel
Austrian monk who is considered the father of modern genetics and discovered many of the basic facts of inheritance.
Inherited features(called characters) come in variety or traits
- A given gene codes for a character but can come in a variety of forms called alleles
- Somatic cells have 2 copies of every gene or allele
- Alleles may be identical so the individual is homozygous or
- Alleles may differ so the individual is heterozygous
- The identity of the alleles an individual possesses is called the genotype while the phenotype is the appearance those alleles give to the individual
- Individuals receive one allele from each parent
- Alleles are divided into separate gametes during meiosis based on the Law of Segregation so that gametes only possess one copy of each alleles
- Somatic cells have 2 copies of every gene or allele
Monohybrid Breeding Experiment
Only one character is examined at a time.
Individuals are considered true breeding if they always produce offspring with a given trait(therefore are homozygous)
- In a classic monohybrid cross, in the P generation, 2 true breeding individuals of different traits are bred and produce all hybrid F1 offspring
- If F1 offspring interbreed, they produce F2 offspring
- If only one of the 2 alleles contributes to the phenotype of a hybrid, it is termed the dominant allele and the other is the recessive allele
- Hybrids are therefore heterozygous
- For complete dominance, in the F2 generation, 3 of 4 offspring will show the dominant trait while 1 of 4 will show the recessive trait seen in one of the original P generation parents
- Genotypic ratio in the F2 generation of a monohybrid cross is always 1:2:1 (AA-homozygous dominant:Aa-heterozygote:aa-homozygous recessive)
- Phenotypic ratio in the F2 generation of a monohybrid cross is always 3:1 of dominant phenotype to recessive phenotype
(True Breeding)An individual who has the dominant phenotype could be homozygous dominant or a heterozygote
- The only way to tell which genotype is present is to conduct a test cross by breeding the dominant phenotype individual with one that shows the recessive phenotype (which is always homozygous recessive)
- If all offspring show the dominant phenotype, the original dominant parent was homozygous dominant
- If ½ of the offspring are dominant phenotypes and ½ are recessive phenotypes, the original dominant parent was a heterozygote
(True Breeding)A dihybrid cross follows 2 characters at once
- Assume for now the genes for the 2 characters are on separate chromosomes
- Due to independent assortment during meiosis I, pairs of chromosomes will segregate into different gametes so that all maternal or all paternal alleles are not inherited together
- This means an individual heterozygous for 2 genes, A and B (AaBb), could produce 4 different combinations of alleles in the gametes
- AB, Ab, aB or ab
- If this individual was crossed with another AaBb, the resulting 16 square Punnett would show a predictable 9:3:3:1 phenotypic ratio
- 9 of the 16 offspring will be dominant for both A and B genes
- 3 of the 16 offspring will be dominant for gene A and recessive for gene b
- 3 of the 16 offspring will be recessive for gene a and dominant for gene B
- 1 of the 16 offspring will be recessive for both gene a and gene b
Variations from Mendel and range of dominance(complete dominance)
Classic Mendelian Genetics
- One allele is completely dominant over the recessive allele
- Heterozygotes will have the exact same phenotype as a homozygous dominant
- F2 genotypic ratio 1:2:1; phenotypic ratio 3:1
Variations from Mendel and range of dominance(Incomplete Dominance)
- Neither allele is dominant over the other
- Heterozygotes have a single, novel phenotype that is intermediate between the homozygotes’ phenotype
- F2 genotypic and phenotypic ratio is 1:2:1
Variations from Mendel and range of dominance(Co-dominance Dominance)
- Neither allele is dominant over the other
- Heterozygotes express both allele’s gene products independently from each other
Variations from Mendel and range of dominance(Multiple Alleles)
- 3 or more variations of a gene can occur at a given gene loci
- Individuals still only have 2 alleles at their loci
Variations from Mendel and range of dominance(Pleiotropy)
- Single gene influences multiple phenotypes
Variations from Mendel and range of dominance(Polygenic Inheritance)
- Multiple genes influence single phenotype
Variations from Mendel and range of dominance(Epistasis)
- 2 or more genes interact with some genes masking phenotypes of other genes
Human Genetics
Recessive disease
- Individuals need to be homozygous recessive to have the disease - Carriers are heterozygotes but do not have the disease
Dominant disease
- Individuals can be heterozygotes or homozygous dominant
dominant and recessive; heterozygous and homozygous; genotype and phenotype.
Dominant: Phenotype that is apparent in a heterozygote
Recessive: Phenotype that is not apparent in a heterozygote
Heterozygous: Having 2 different alleles for a given character
Homozygous: Having 2 copies of the same allele for a given character
Genotype: The actual genetic makeup, what alleles are present
Phenotype: The appearance the alleles give to the organism for a given character
Dominant
Phenotype that is apparent in a heterozygote
Recessive
Phenotype that is not apparent in a heterozygote.
Heterozygous
having 2 different alleles for a given character.
Homozygous
Having 2 copies of the same allele for a given character.
Genotype
The actual makeup, what alleles are present.
Phenotype
The appearance the alleles give to the organism for a given character.
How does a testcross help determine if an individual with the dominant phenotype is homozygous or heterozygous
The dominant phenotype can either be homozygous dominant or a heterozygote and you can not tell by looking at them. A test cross is a mating between the ambiguous dominant phenotype and a recessive phenotype that you know must be homozygous recessive. If you get any offspring from this mating that have the recessive phenotype, you know the dominant phenotype individual was a heterozygote. If you get only dominant phenotype offspring, you know the unknown dominant phenotype was homozygous dominant.
Mendel’s law of independent assortment and the behavior of chromosomes during meiosis
In dihybrid or higher crosses, each pair of alleles segregates independently during gamete formation if they are on different chromosomes. This occurs at
the metaphase I step of meiosis I when the homologues line up at the metaphase plate. It is completely random which side each homologue picks with regards to maternal or paternal origin. This gives many different variations of gametes, with
2n possibilities.
Example: Monohybrid cross between Aa (mom) x Aa (dad): What’s the chance of getting an offspring that is Aa?
Rule of multiplication:
- Chance of Aa (mom) giving an “A” = ½
- Chance of Aa (dad) giving an “a” = ½
- Multiple: ½ x ½ = ¼
But need Rule of addition also since mom could have given “a” and dad given
- “A”: ½ x ½ = ¼
Need to add first ¼ + second ¼ to give overall ½ chance of getting Aa from this mating.
Differences of phenotypic expression of heterozygote(come, incomplete, and co-dominance)
Complete dominance: Heterozygote will have the dominant phenotype only
Incomplete dominant: Heterozygote will have a unique phenotype that is neither the dominant nor recessive
Codominance: Neither allele is considered dominant nor recessive. Both alleles manifest themselves
Describe the inheritance of the ABO blood system
There are 3 alleles (multiple alleles): A (IA), B (IB) and O (i)
The A and B alleles are dominant over the O. A and B are codominant to each other. This gives 6 possible genotype and 4 possible phenotypes.
Genotype = Phenotype IAIA or IAi = Type A IBIB or IBi = Type B IAIB = Type AB ii = Type O
Understanding inheritance of human genetic diseases and how “carriers” function
Many human diseases are inherited by simple Mendelian complete dominance and follow the same genotypic and phenotypic ratios predicted by Mendel. Depending on whether or not the disease is a dominant or recessive disease will influence if the person has symptoms of the disease.
If it’s a dominant disease, the heterozygote as well as the homozygous dominant genotype will have the disease. Many dominant genetic diseases are not passed on between parents and offspring but are instead result of spontaneous mutations in the offspring that can then be passed on if the disease is not a fatal one.
If it’s a recessive disease, only the homozygous recessive genotype will have the disease phenotype. Individuals that are heterozygotes for a recessive disease are called carriers since they possess the abnormal allele and can pass it on to offspring but they themselves do not show symptoms of the disease.
