Genetics Heredity Blending Theory of Inheritance Until early

Genetics & Heredity

Blending Theory of Inheritance • Until early 1900 s, people believed in the “blending theory of inheritance” • Children look like a mix of their parents traits

• But sometimes…

• Or even…

Gregor Mendel “Father of Genetics” • late 1860 s • scholarly monk who studied garden peas • garden peas have either purple or white flowers

Mendel’s Three Laws of Inheritance 1. Law of Dominance – homozygous parents with contrasting traits produce offspring that resemble ONE parent

2. Law of Segregation – every individual has 2 different genes for each trait, during meiosis the genes are sorted into different gametes

3. Law of Independent Assortment – each trait is inherited as a separate unit, independent of other traits

Remember: In the mid to late 1800 s, Mendel knew nothing of DNA, chromosomes, or genes! – DNA was first identified in the late 1860 s by Swiss chemist Friedrich Miescher. – In 1952, Franklin and Wilkins took the X-ray diffraction image of crystallized DNA – In 1953, Watson and Crick described the structure of DNA

• Mendel crossed purple and white flowers that were “purebred” • Offspring were always purple

• But when he crossed the purple offspring (F 1)… • He got purple AND white flowers! • 75% (3/4) were purple • 25% (1/4) were white

• Parents each have two copies of the gene for a trait – Stored on different chromosomes (Law of Segregation) – Each copy is called an allele – We use letters to name the alleles • have two (one from mom & one from dad) with different letters • Peas could be PP, Pp, or pp Purple flowers: P White flowers: p

• Homozygous – two of the same allele (PP or pp) • Heterozygous – two different alleles (Pp) • Dominance – one allele masks the other (capital letter) – the allele that is not dominant is called recessive (lower case letter) • PP – (homozygous dominant) • Pp – (heterozygous dominant) • pp – (heterozygous recessive)

• The set of alleles is called the genotype – gen = genetics (what genes do you have? ) – homozygous dominant (PP) – heterozygous dominant (Pp) – homozygous recessive (pp) • The physical traits the genotype results in is called the phenotype – ph = physical (what do you look like? ) – purple – white

• When creating gametes (eggs or sperm by meiosis), diploid cells produce haploid cells (half the genetic info) • Each gamete only gets one allele for a gene (Mendel’s Law of Segregation) • Ex) A purple flower (Pp) creates two gametes – the first has a dominant allele (P) – the second has a recessive allele (p)

Punnett Squares (How we solve genetics problems!) There are two different types of punnett squares: monohybrid and dihybrid: • A monohybrid cross finds the possibilities of only one trait. – Ex) What colour will the pea be? • A dihybrid cross finds the possibilities of TWO traits. – Ex) What will the pea color AND texture be?

Punnett Square Male gametes haploid (n) (One allele per sperm ) Possible offspring diploid (2 n) Female gametes haploid (n) (One allele per egg) (Two alleles per zygote)

Examples: What if we crossed a PP (purple) flower with a pp (white) flower? How many of the offspring are purple and how any are white?

What if we crossed a Pp (purple) flower with a Pp (white) flower? How many of the offspring are purple and how any are white?