1. The Genetics of Inheritance

2. The Genetics of Inheritance
genetics is the branch of biology dealing with the principles of variation and inheritance
inherit means that certain traits are transmitted from one generation to the next
traits are distinguishing characteristics that make an individual unique
genes are the part of the chromosome that governs the expression of a particular trait
alleles are different available versions of the same gene - e.g., eye colour gene comes in brown and blue

3. Some Early Ideas Hippocrates (460-377 B.P.)
every part of the body was involved in the production of “seeds”
the seeds would fuse together to make a new person
Aristotle ( B.P.)
male and female semen mixed upon conception (female important)
William Harvey ( )
new individuals arose through the process of epigenesis
embryo grew in stages and was affected by factors both inside and outside of the mother

4. Some Early Ideas Anton van Leeuwenhoek (1632-1723)
used a microscope to examine gametes
found “animalcules” in semen and decided they were preformed embryos
female contribution was only the influence of the uterus during embryo development
during the 1800’s, the blending theory became popular
sperm and egg mixed together resulted in offspring that were a blend of the parent’s characteristics
this theory predicted that a plant with red flowers crossed with a plant with white flowers would produce offspring with pink flowers

5. Some Early Ideas Francis Galton (1870’s) (cousin of Darwin)
disproved the theory of pangenesis (still influenced biology 100 years ago)
genes formed in every organ of both the female and male body and moved to the gonads   
Charles Darwin ( )
offspring had variations of their parents characteristics
couldn’t explain why – suggested current science had not yet discovered the puzzle pieces yet

6. The Inheritance of One Trait
Gregor Mendel ( ) – Father of Genetics
studied the common garden pea (Pisum sativum) and determined the fundamental principles of genetics   
Proposed Mendel’s laws of heredity
peas exhibit different, measurable characteristics that express themselves as one of two traits (ex. characteristic for height could be tall or short)   

7. The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross
Mendel bred tall plants together until the offspring were always tall – a true breeding stock (purebred)
the true bred plants became the parent or P generation   
he then crossed a true bred tall plant with a true bred short plant
the offspring of this cross was the first filial generation or F1 generation   

8. The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross
all of the F1 plants were tall which indicated that the trait for tall plants must be dominant over short
dominant traits are always expressed and recessive traits are latent (present but inactive)
he formulated the principle of dominance: when individuals with contrasting traits are crossed, the offspring will express only the dominant trait
Mendel showed that heredity is not just a blending of traits   

9. tall x short all F1 were tall
The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross
tall x short all F1 were tall   
letters are used to represent the different alleles
T – tall (dominant) t – short (recessive)
parent cross TT x tt
all F1 generation Tt
(tall and short are the
different possible alleles)   
if both of the alleles are the same, the individual is said to be homozygous
if the alleles are different, heterozygous   

10. The Inheritance of One Trait Mendel’s First Experiment: The Monohybrid Cross
TT – homozygous tall plant
Tt – heterozygous tall plant
tt – homozygous short plant   
this type of cross is called a monohybrid cross because only one trait (height )was studied   

11. The Inheritance of One Trait Mendel’s Second Experiment: The Monohybrid Cross Part Two
Mendel then bred the F1 generation together and found that 3 out of 4 plants in the F2 generation were tall while one was short
F2 resembled one parent from the P generation 75% of the time and the other 25% of the time
this 3:1 ratio is know as the Mendelian Ratio
this information led to the formation of the Law of Segregation which states that inherited traits are determined by pairs of ‘factors’ and these factors segregate (separate) in the gametes, with one in each gamete (pollen and ovules in plants)

12. the F2 generation was different
The Inheritance of One Trait Mendel’s Second Experiment: The Monohybrid Cross Part Two
of course, factors were actually alleles, where one is dominant over the other
the F1 generation had a chance of getting only a T from the tall plant and a t from the short plant resulting in plants that were all heterozygous (Tt)
the F2 generation was different   
3:1 Mendelian Ratio   
tall x tall tall tall tall short   

13. Punnett Squares T T T t t Tt Tt T TT Tt t Tt Tt t Tt tt
Punnett squares are used to organize the possible trait combinations and to calculate the probability of inheriting a particular trait
it can also tell you the genotype (genetic makeup of an individual) and the phenotype (the appearance of a trait in an organism)
P generation cross
F1 generation cross
father
mother
father
mother T T F2
genotypes:
TT, Tt, tt
phenotypes:
tall, short
genotypic ratio 1:2:1
phenotypic ratio 3:1 T t t Tt Tt T TT Tt t Tt Tt t Tt tt

15. Class Activity
Dominant and Recessive Traits

16. Pedigrees Experimental crosses are not done on humans
Scientists study family trees or pedigrees to trace genetic diseases passed on from generation to generation
Pedigrees reflect phenotypes

17. Human Pedigree
This shows a brother and sister affected by a disease, along with their maternal grandmother