Chapter 12: Mendel’s Experiments and Heredity

one belief about inheritance during the 1600-1700s was homunculus, which suggests that

sperm contained fully formed “little people”

one belief about inheritance during the 1600-1700s was maternal heredity, which suggests that

females controlled all traits

one belief about inheritance during the 1600-1700s was paternal heredity, which suggests that

males controlled all traits

one belief about inheritance during the 1600-1700s was blending inheritance, which suggests that

offspring were the average between both parents (.e.g, if mom had blonde hair and dad had black hair, the offspring will have brown hair

Mendel chose garden peas because

they were easy to grow, came in readily distinguished varieties, and their reproduction could be controlled

Mendel focused on

flower color (white or purple) and seed color (yellow or green)

Mendel performed self-fertilization on pea plants, which is

crossing plant with itself; self-cross

stamens

plant structures that releases sperm-producing pollen

Mendel removed stamens from

the purple flower (parent, P of true breeding plants)

carpel

plant structure that produces eggs

Mendel added pollen from (1) to (2)

white flower stamens, purple flower carpel

Mendel concluded that

purple flower trait is dominant over the white flower trait

Mendel’s1st Law of Inheritance

alternative versions of genes (alleles) account for variations in inherited characters (heritable features that vary among individuals)

Mendel’s 2nd Law of Inheritance

for each inherited character, an organism inherits 2 alleles (homozygous and heterozygous) from each parent

homozygous

organisms with two identical alleles for a gene (e.g., AA or aa)

heterozygous

organisms with two different alleles for a gene (e.g., Aa)

Mendel’s 3rd Law of Inheritance

one allele for a heterozygous gene determines the organism’s appearance (dominant allele; e.g., A) and the other has no noticeable effect on the organism’s appearance (recessive allele; e.g., a)

trait

a variant of a characteristic (e.g., flower color, stem length, etc.)

true breeding (purebred)

when parents always produce offspring having the same traits

hybrids

offspring of two different purebred varieties (e.g., offspring from one white flower and one purple flower)

phenotype

an individual’s observable trait (e.g., purple flower, white flower)

genotype

specific combination of “hereditary particles” carried by an individual that causes a phenotype

gene loci

physical location of a gene within the chromosome

monohybrid cross

a cross between 2 hybrids for 1 trait controlled by 1 gene

for the product (ADD) rule of probability, you

multiply the individual probability of each event occurring alone

for the additive (OR) rule of probability, you

add the overall probability of the individual events

dihybrid cross

a cross between 2 organisms with heterozygous traits to see if the traits are independent or not

a test cross is used to

determine whether an organism expressing a dominant trait is homozygous or heterozygous

incomplete dominance

heterozygous organisms have an intermediate phenotype that’s different from the parents (e.g., snapdragons have dominant homozygous = red flowers; recessive homozygous = white flowers; and heterozygous = pink flowers, instead of red like traditional Mendelian genetics)

multiple alleles and codominance

two alleles are codominant if they’re expressed simultaneously in heterozygous individuals (e.g., in the ABO blood group, alleles IA and IB are codominant (AB blood group), and the three possible alleles are IA, IB, and ii (recessive)

sex-linked traits

genes located on a sex chromosome (mostly X chromosome)

lethal alleles

alleles that are disease causing when homozygous (e.g., AA for achondroplasia)

polygenetic inheritance

traits controlled by multiple genes, creating more complex punnet square combinations

epigenetic inheritance

transmission of traits not directly involving DNA sequence

histone modifications

a type of epigenetic inheritance caused by enzymes adding chemical modifications to histones, affecting whether a protein coding gene will be expressed

chemical changes to chromosomes can

be reversed

pedigree

a tool used to observe family tree within a family lineage to understand inheritance