Heredity
Name Naren
The passing of traits from parent to offspring is heredity. The scientific study of heredity is genetics.
The father of genetics is mendel. He used pea plants and figured out the rules of trait inheritance. He
said there are 2 factors for every trait and that each parent contributes 1 factor. The theory of
probability says each genetic event is independent of the previous event. The Punnett square is a
tool, which tells us all the possible outcomes of a genetic cross.
Vocabulary
1.The factor that controls a trait is a gene. They are located on DNA
strand in the nucleus the cell.
2.The different forms of one gene are called alleles
3.There are two types of alleles: dominant and recessive.
4.The dominant always shows up in an organism when it is present. The recessive allele is
masked by the other trait. It will only show up when there are 2 of them.
5. If an organism has two identical alleles it is homozygus, If an organism has one of each allele it is
heterozygus .
6. An organism’s phenotype is its physical appearance or its visible traits. An organism’s genetic makeup is its
genotype .
7. In incomplete dominance neither allele is dominant and they both appear in the heterozygous organism.
8. In codominence neither allele is dominant and they blend to make a new trait in the heterozygous individual.
9. In sex-linked traits, the gene for a trait is carried on a chromasome, usually the 22 chromosome.
Gender Inheritance
Human body cells have 46 chromosomes. Human sex cells have 23 chromosomes. The gender of a child is
determined by the sex chromosomes.
Females are XX and males are XYt.
The male sex cell determines the gender
of the child. Draw a Punnett square that shows
gender inheritance- show the genotype and phenotype ratio.
Practice crosses:
Determine the phenotype and genotype percentage using a punnett square.
Purple dominant over white in flower
cross homozygous purple with heterozygous
Tall incomplete dominant with short in plant to make medium cross tall with medium
Brown co-dominant with white in cows to make roan
cross two roan cows
Human Inheritance
Some traits are caused by a single gene and are inherited according to Mendel’s rules. An example would be
autosomal dominiant. Some traits are controlled by more than one gene, these are called autosomal recessive.
An example would be eye color
Other traits are controlled by a single gene that has more than one allele. These are called multiple alleles. An
example in humans is our blood. There are 3 alleles for this trait. They are A , B, and O. This creates the four
blood types ( list genotype):
A blood = AAor AO
AB blood =AB
B blood = BBor BO
O Blood = OO
In sex-linked traits, the gene for a trait is carried on a Sex chromasome, usually the 23
chromosome. In females the recessive allele on one sex chromosome is masked by the
dominant allele on the other sex chromasome. In males there is no corresponding dominant
allele on the sex chromosome, so the male has the trait. An example of a sex-linked trait is
.