1. Principles of Heredity
Exercise 45
Principles of Heredity

2. Objectives Allele, gene Genotype, phenotype Homozygous, heterozygous
Recessive, dominant, incomplete dominance
Sex-linked traits
Punnet Squares

3. Genetics Gene: a hereditary, functional segment of DNA
Chromosomes contain DNA
Each chromosome contains many genes
Each gene has instructions for constructing a specific protein
Pair of chromosomes
GENE
GENE

4. Genotype
Your 46 chromosomes in each somatic cell & the genes present on them
Combination of your parents’ genotypes
Fig

5. Phenotype Genotype = the “blueprint or plans”
The instructions found in your genotype determine your anatomical and physiological characteristics, your phenotype.
Hair color, eye color, foot size, etc.
Genotype = the “blueprint or plans”
Phenotype = the finished “building”

6. Genetics Alleles: various forms of any one gene Allele for brown eyes
Two chromosomes in a pair can have different alleles for the one gene
EXAMPLE:
This gene controls the trait eye color
From Mother
From Father
Allele for blue eyes

7. If both chromosomes of the pair
carry the SAME allele of a particular gene,
you are HOMOZYGOUS for the trait affected by that gene.
This allele will be expressed in your phenotype:
Allele for brown eyes
From Mother
You have brown eyes
From Father
Allele for brown eyes

8. If each chromosome of the pair
carries a DIFFERENT allele of a particular gene,
you are HETEROZYGOUS for the trait affected by that gene.
Allele for brown eyes
From Mother
You have
???
eyes
From Father
Allele for blue eyes

9. If you are HETEROZYGOUS for the trait affected by that gene, your phenotype is determined by recessive and dominant alleles:
A DOMINANT allele will be expressed in the phenotype, regardless of what the other allele is.
*denoted by uppercase letters
A RECESSIVE allele will be expressed in the phenotype ONLY if the same allele is present on BOTH CHROMOSOMES of a homologous pair.
*denoted by lowercase letters

10. If you are HETEROZYGOUS for the trait affected by that gene, your phenotype is determined by recessive and dominant alleles:
Eye color:
Brown = dominant
Blue = recessive
Allele for brown eyes
From Mother B
You have BROWNeyes
From Father b
Allele for blue eyes

11. If you are HOMOZYGOUS for the trait affected by that gene, your phenotype is determined:
Eye color:
Brown = dominant
Blue = recessive
Allele for brown eyes
From Mother B
You have
BROWN eyes
From Father B
Allele for brown eyes

12. If you are HOMOZYGOUS for the trait affected by that gene, your phenotype is determined:
Eye color:
Brown = dominant
Blue = recessive
Allele for blue eyes
From Mother b
You have
BLUE eyes
From Father b
Allele for blue eyes

13. INCOMPLETE DOMINANCE:
Heterozygous alleles produce a phenotype that is different from both phenotypes when the alleles are homozygous
Sickle-cell anemia:
Homozygous “A” alleles  normal shape RBCs
Homozygous “S” alleles  sickled RBCs
Heterozygous AS  do not develop anemia, but have RBCs that will sickle in low oxygen levels AA SS AS A
From Mother
You have
both
RBCs
From Father S

14. Sex Chromosomes Appear different from each other X chromosome—larger
Carried by all eggs
Carried by some sperm
Y chromosome—determines the male sex
Carried by some sperm, no eggs
Normal female pair: XX
Normal male pair: XY
Fig

15. Sex-Linked (X-linked) Traits
In most cases, there’s no corresponding allele on the Y chromosome
Color-blindness
Normal Vision
Dominant allele (C)
on X chromosome XC
Red-Green Color-blindness
Recessive allele (c)
on X chromosome Xc

16. Sex-Linked (X-linked) Traits
Normal vision
Color-blind
Females: XCXc heterozygous
XCXC homozygous dominant
XcXc homozygous recessive
Males: XCY heterozygous
XcY homozygous

17. MATERNAL ALLELES (for eye color) PATERNAL ALLELES (for eye color)
Punnet Squares
Box diagram depicting which alleles come from each parent
Allows prediction of the offspring’s genotypes and phenotypes
MATERNAL ALLELES (for eye color)
b b
All offspring (100%) from homozygous dominant father & homozygous recessive mother will be HETEROZYGOUS for the trait. Bb B
PATERNAL ALLELES (for eye color)

18. Genotypes, Phenotypes Bb Genotype: Bb Phenotype: Brown eyes
Eye color:
Brown = dominant
Blue = recessive
Genotype: Bb
Phenotype: Brown eyes
MATERNAL ALLELES
b b Bb B
PATERNAL ALLELES

19. Genotypes, Phenotypes BB Bb Genotype: BB or Bb Phenotype: Brown eyes
Eye color:
Brown = dominant
Blue = recessive
Genotype: BB or Bb
Phenotype: Brown eyes
MATERNAL ALLELES
B b BB Bb
50% offspring will be HETEROZYGOUS for the trait.
50% offspring will be HOMOZYGOUS dominant. B
PATERNAL ALLELES

20. Genotypes, Phenotypes Bb bb Genotype: Bb or bb
Eye color:
Brown = dominant
Blue = recessive
Genotype: Bb or bb
Phenotype: Brown eyes or
Blue eyes
MATERNAL ALLELES
b b Bb bb
50% offspring will be HETEROZYGOUS for the trait.
50% offspring will be HOMOZYGOUS recessive. B b
PATERNAL ALLELES