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Terminology, Mendel's experiments, Monohybrid crosses, dihybrid crosses, etc


- segment of DNA
- code for polypeptide synthesis (protein)


- sing. locus
- location of a specific gene on a chromosome


- alternative forms of the same gene (sometimes there may be the genes are the same), located at the same position (locus) on homologous chromosomes
- pairs of alleles are related to homologous pair of chromosomes (1 from mom and 1 from dad).


- separation of the maternal and paternal alleles during Meiosis I (in the separation of homologous pairs)
- Mendel's First Law

Independent Assortment

- random distribution of genes into the games during Meiosis due to crossing over/recombination
- Mendel's 2nd Law

Linked Genes

- 2 gene loci that are extremely close together will not independently assort, therefore the two genes will not independently cross over
- exception to the rule of independent assortment


- alleles at a particular locus that are identical ie) RR, or rr
- meiosis will produce identical gametes for this locus


- alleles at a particular locus are not identical ie) Rr
- meiosis will produce different gametes for this locus


- sex chromosomes that are present in males
- no corresponding allele ie) XY (not the same, there is only allele per pair)


- types of alleles at a particular locus
- genetic makeup


- physical expression observed for a particular gene at a certain locus ie) red hair

Dominant Trait

- allele in a heterozygous state that is expressed
ie) Pp = purple flower because purple is dominant to white

Recessive Trait

- allele in a heterozygous state that is masked.
ie) Pp = purple flower because white is recessive to purple

Gregor Mendel

- father of classical genetics
- conducted garden pea experiments
- results led to understanding of inheritance of single gene traits

True Breeding

- will always get the same offspring phenotype from matings with homozygous alleles at a particular locus.
- ie) RR and rr, where R ex[resses Red hair and r expresses brown hair, offspring will have Rr but express Red Hair phenotpyically

Monohybrid Cross

- mating in which we are following the inheritance of 1 locus
- ie) pea color

Dihybrid Cross

- mating in which we are following the inheritance of 2 loci
- ie) pea color and pea shape

Characteristics of Monohybrid Crosses

1. alleles segregate during game production
- gametes receive 1 allele per locus
- explanation: homologous chromosomes separate in meiosis II
2. genes do not blend.
- gene structure do not change from one generation to the next
3. genes can interact as recessive or dominant
- gene expression may be masked, due to some forms of the gene being recessive.

Characteristics of Dihybrid Crosses

1. Loci assortment into gametes independently of all other loci (one locus does not affect the other)
- chromosomes move to opposite poles independently of each other in Anaphase II
- loci on the same chromosome still assort independently do to crossing over

Multiple alleles

some loci have more than 2 possible alternatives


- two alleles that express themselves completely when heterozygous
- ie) ABO blood type:
IAIA or IAi = A blood Type
IBIB or IBi = B blood type
ii = O blood type
IAIB = AB blood type

Incomplete Dominance

- alleles in the heterozygous state will show an intermediate phenotype
- ie) pink snap dragons are a hybrid between dominate and recessive phenotypes

Mendel's Experiment of Monohybrid Crosses

1. P1 generation: parental generation = smooth and wrinkled peas
2. F1 generation: offspring of P1 = all smooth
3. F2 generation: offspring of F1 = 5,474 smooth and 1,850 wrinkled (Phenotype Ratio: 3 Purple:1 white, Genotype Ratio: 1 PP:2 Pp:1 Pp)

Mendelian Inheritance with Monohybrid crosses

1. Genes determine traits and can be present but not expresses
2. Despite the identical appearance of P1 and F1, they were genetically different
3. The F1 generation carried genes for smooth and wrinkled traits because both types were present in F2 generation
4. F1 generation carried two genes for one trait, then P1 and F2 also carried two genes for the trait as well.

Mendelian Inheritance on Dihybrid Crosses

1. P1 Generation: Parental Generation = SSYY and ssyy, ssYY and SSyy
2. F1 Generation: Parental Offspring = all SsYy smooth yellow
3. F2 Gneration: F1 Offpsring = 9 smooth yellow:3 smooth green:3 wrinkled yellow:1 wrinkled green (phenotypes)

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