Genetics, Heredity and Punnett Squares

Why / How do populations change over time? GENES! Genetics is the science of heredity (the passing on of genetic material from parent to offspring) Gregor Mendel began studying heredity around 1850 using common pea plants.

Mendel realized that organisms, such as peas, have “factors” that control each trait they pass on to offspring. Beginning with Mendel’s work, and including the work of many other scientists, we now know: Gene- a heritable factor (segment of DNA) that determines a specific trait (by coding for the production of a specific protein). Genes (DNA) are carried on structures called chromosomes. Chromosomes are made up of protein and “supercoiled” DNA (so it can all “fit” in our cells).

Genes come in different forms! There are different forms of genes, called alleles. Alleles can be dominant (stronger) or recessive (weaker) Dominant and recessive alleles for the SAME trait are represented by the same letter: Dominant = upper case Recessive = lower case The letter you use is the FIRST letter of the name of the dominant trait! Example: R = round seeds, r = wrinkled

The genes aren’t always what you see! “What you SEE” (the physical expression of a trait) is an organism’s phenotype (example: tall pea plants). The combination of alleles an organism has for a trait is its genotype. (example: Tt) – Because organisms get one allele for each trait from each parent, a genotype is made up of TWO letters (one from mom and one from dad). Genotypes are: – Heterozygous: made up of different alleles (Tt) Note: Heterozygous individuals (Nn) are carriers: they CARRY recessive alleles but do not show them in their phenotype due to the presence of a dominant allele (important in sex-linked traits and genetic diseases) – Homozygous: made up of the SAME alleles (TT or tt) If both alleles are dominant, the genotype is called homozygous dominant (TT) If both alleles are recessive, the genotype is called homozygous recessive (tt)

Genetics studies rely on 3 rules that Mendel established Rule of Dominance – when a dominant allele is present that trait is expressed (it WILL be seen) Law of Segregation – every individual has 2 alleles of each gene and, each gamete (sex cell – sperm and egg) randomly receives one of these alleles Law of Independent Assortment – genes for different traits are inherited independently of each other (example: just because you have blonde hair doesn’t mean you’ll always have blue eyes).

Predicting Genotypes and Phenotypes in Offspring Using a punnett square, place dad’s alleles on top (one over each box) and mom’s alleles on the left-side (one next to each box). Fill in each box and see the possible results. Example: Two heterozygous tall pea plants are crossed. a. What are the possible phenotypes of their offspring? ¾ = 75% tall ; ¼ = 25% short b. What are the possible genotypes of their offspring? ¼ = 25% TT (homozygous dominant); 2/4 = 50% Tt (heterozygous); ¼ = 25% tt (homozygous recessive)

Make a Punnett Square

Complex Patterns of Inheritance Incomplete Dominance – in some cases, a phenotype (appearance) occurs for heterozygous individuals that is a blend of the two alleles Example: a homozygous red flower (RR) crossed with a homozygous white flower (rr) produces all pink flowers(red mixed with white = Pink = Rr)

Codominance Codominance – both alleles are equally strong, so they BOTH show in the phenotype Example: In some chickens the alleles for black- feathers and white-feathers are codominant (both dominant/ equally strong). A heterozygous chicken (BW) will SHOW both black and white feathers!

Sex-linked Inheritance Humans have 23 PAIRS of chromosomes The 23 rd pair determines the sex of the individual Females = two X chromosomes (XX) Males = one X and one Y chromosome (XY) Traits controlled by genes on sex chromosomes are called sex-linked The X and Y chromosomes are not the same (they are not homologous) so they do not carry alleles for the same genes Sex-linked traits are more common in males: Only one X chromosome (whatever inherit will show – no other chromosome to overpower) Hemophilia and color blindness on X-chromosome; Males inherit X from their mothers; IF recessive allele (for hemophilia or color blindness etc.) is on that X, the man WILL have the condition (X n Y) ONLY Females can be CARRIERS of sex-linked traits (two X chromosomes so can be heterozygous – a dominant allele on one X and a recessive allele on the other: X N X n )

There can be Multiple Alleles for a Trait There are 3 alleles for blood type: I A, I B, i A and B are both dominant to i, and A and B are codominant with each other Type A blood: I A I A or I A isurface molecule A Type B blood: I B I B or I B isurface molecule B Type AB blood: I A I B surface molecules A&B Type O blood: iino surface molecule If a man is type O and a woman is type A, what blood types could their offspring have?

Antigens (Surface molecules) and Antibodies in Blood