Chapter 10: Mendelian Genetics and Meiosis
Gregor Mendel- a monk with a lot of spare time
1st person to study heredity quantitatively
heredity- the passing on of characteristics from parents
to offspring
genetics- the study of heredity
traits- characteristics that are inherited
Mendel was able to predict how and what traits would be passed on. Worked with garden peas.
Why peas???
They reproduce sexually- produce gametes
Remember: Pollenation: transfer of pollen to pistil
Fertilization- union of gametes
Sex organs in same flower tightly enclosed in petals
therefore self pollination likely
when cross pollination necessary, did it manually
Therefore, he always knew the parents
The term to cross means to breed
The key to his success?
He started out studying one trait at a time
Used math to quantify outcomes of crosses
Used true breeding tall and short plants
The first experiment: A Monohybrid Cross
Monohybrid cross =the parents differ in only one trait
Cross a tall plant with a short plant ----à hybrid plant
P1 Short x Tall
(Cross pollentation)
F1 Tall Tall Tall Tall
(Self Pollenation)
F2 3 Tall 1 Short
Mendels Conclusion (after thousands of crosses, and a whole lot of counting): each organism has 2 factors that control each trait.
These factors are _________, on __________
Genes exist in alternate forms, called _______
Dominant Trait- the one that shows in F1, masks other trait
Recessive Trait- disappears in F1, reappears in F2, is masked
by a dominant trait
Remember that traits are coded for by DNA in genes, so you have dominant and recessive alleles.
Therefore, the genotype tall plants could be either TT or
Tt, and short plants must be tt
(dominant is capitalized, recessive is lower case)
Law of Segregation
In F1,all the plants were tall, in fact Tt (TT x tt)
Each F1 plant produces 2 gametes with different alleles: T
and t
When fertilization occurs, genes get paired randomly
Therefore, The Law of Segregation states that:
Phenotypes and Genotypes
Tall plants may look the same, but they arent
Some tall plants only have tall baby plants = _____________
Some tall plants can make tall and short babies
Phenotype:
Genotype:
Homozygous:
Heterozygous:
Dihybrid Crosses
Parents differ in ___ traits
P1 Round/Yellow (RRYY) x Wrinkled/Green (rryy)
F1 Round/Yellow (_____)
F29 Round/Yellow 3 Round/Green3 Wrinkled/Yellow 1 Wrinkled/Green
Therefore, ______ and _____ are dominant to ______ & ____
Law of Independent Assortment
Different traits are inherited independent of each other, which means that alleles sort into gametes indep. of each other (the combinations of traits in an offspring is up to chance)
Punnett Squares
Short hand way to figure expected proportions of possible genotypes in offspring
Monohybrid Cross
Dihybrid Cross
Meiosis
Genes are lined up on __________
Peas have 14 chromosomes: 7 homologous pairs (1 from
each parent)
Diploid
Haploid
The combinations of the alleles on the two homologous chromosomes determines what an organism looks like- they encode the same _____, but different ______
Why Meiosis?
Mitosis: 2n -à 2n
If mitosis were the only form of cell division:
There has to be a way to segregate so that each gamete ends up as 1n= Meiosis
Meiosis: cell division that produces gametes with ½ the
number of chromosomes as somatic cells
only happens in germ cells
sexual reproduction
Meiosis I Meiosis II
1 x 2n -------------------------à 4 x 1n
Phases of Meiosis
Interphase-
Prophase I
Metaphase I
Anaphase I
Telophase I
Cells are now 2n, need one more cell division to get to 1n
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
End Result:
Meiosis provides genetic variation:
shuffles chromosomes
during meiosis, chromosomes line up at equator
In peas, 7 chromosomes, 2 ways for each to line up
Increases as # chromosomes increases
In humans, n=23, therefore possible combinations in each gamete
Plus, chromosomes can recombine- CROSSING OVER
Meiosis explains Mendels results- 1 allele for each trait in each gamete, and independent assortment
Mistakes in Meiosis
Nondisjunction-chromosomes dont separate correctly,
Both chromosomes (tetrad) move to one pole in
meiosis I
Trisomy
Monosomy
Triploidy
