Dominant & Recessive Traits Chapter 12

DOMINANT TRAITS. RECESSIVE TRAITS eye coloring. brown eyes DOMINANT TRAITS RECESSIVE TRAITS eye coloring brown eyes grey, green, hazel, blue eyes vision farsightedness normal vision normal vision normal vision normal vision nearsightedness, night blindness, color blindness* hair dark hair blonde, light, red hair non-red hair red hair curly hair straight hair full head of hair baldness* widow's peak normal hairline facial dimples no dimples features unattached earlobes attached earlobes freckles no freckles broad lips thin lips

Dominant Trait Recessive Trait Appendages extra digits normal number fused digits normal digits short digits normal digits fingers lack 1 joint normal joints limb dwarfing normal proportion clubbed thumb normal thumb double-jointedness normal joints bent pinky straight pinky other immunity to poison ivy susceptibility to poison ivy normal pigmented skin albinism normal blood clotting hemophilia* normal hearing congenital deafness normal hearing & speaking deaf mutism normal- no PKU phenylketonuria (PKU) *sex-linked traits

Longer 2nd toe is dominant over 2nd toe shorter than big toe.

Monohybrid cross- studies 1 trait Mendel’s experiment Gregor Mendel conducted experiments that used monohybrid crosses; carried out in 3 steps Monohybrid cross- studies 1 trait Step 1: Mendel allowed pea plants to self-pollinate for several generations to get offspring that are true-breeding (purebred). He used true-breeding plants as the first generation in his experiment. These were called the parental (P) generation

Mendel’s experiment Step 2: Mendel crossed 2 P generation plants with contrasting traits. This produced offspring called the first filial generation (F1 generation). Mendel recorded the # of F1 generation plants with each trait. Step 3: Mendel allowed the F1 generation to self-pollinate, producing a second filial (F2) generation. He then recorded the # of F2 generation plants with each trait.

Notice this # is approximate. Why? Mendel’s Results F1 generation- 100% displayed the same trait for a given character (the dominant trait) F2 generation- the trait that disappeared in the F1 generation reappeared. The ratio of the characters in this generation were approximately 3:1 (dominant:recessive). Notice this # is approximate. Why?

Law of Segregation When an organism produces gametes, each pair of alleles is separated and each gamete has an equal chance of receiving either one of the alleles.

Patterns in Inheritance? Mendel conducted another experiment to see if traits were passed down in a pattern. He did this by conducting a dihybrid cross. dihybrid cross- involves 2 characters He found that the inheritance of one trait did not affect the inheritance of any other trait. This is because of the Law of Independent Assortment.

Law of Independent Assortment During gamete formation (meiosis) the alleles of each gene segregate independently. Alleles can “mix and match” i.e. Round seeds can be green or yellow (as can wrinkled seeds)

Exceptions: Genes that are located close together on the same chromosome will rarely separate independently. These genes are linked genes.

Sex Linked Traits Genes located on one of the sex chromosomes (X or Y) Most sex linked traits are on the X chromosome because it is much longer than the Y chromosome More common in males since they only have one X chromosome Examples: colorblindness, hemophelia

More complicated traits Most traits do not follow Mendel’s pattern of inheritance because most genes either have more than 2 possible alleles OR are controlled by more than 1 gene. Examples: Polygenic inheritance Incomplete dominance Multiple alleles Codominance

Polygenic Inheritance Characters that are controlled by more than one gene are polygenic traits. Examples: eye color (amount of greenness or browness of the eye), height, skin color

Incomplete dominance 2 dominant alleles occur. Neither allele is completely dominant over the other. Example: snapdragons

Multiple Alleles Genes that have 3+ possible alleles have multiple alleles. Only 2 alleles for a gene can be present in one individual. The determination of dominance in these cases can be very complex. Example: blood type (alleles= IA, IB, i)

Examples: applies to blood type AB, roan fur color Codominance 2 traits can appear at the same time for some characters, leading to codominance. In these cases, both alleles for the same gene are fully expressed. Examples: applies to blood type AB, roan fur color in cattle

Pedigree A family history that shows how a trait is inherited over several generations. A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next. A pedigree can help us learn about sex-linked traits, dominance, & heterozygosity.

Affects of the environment Some phenotypes can be affected by conditions in the environment (such as nutrients & temperature). Examples: color of the arctic fox is affected by temperature, height in humans can be affected by nutrition, exposure to too much oxygen in premature babies leads to blindness while too little oxygen leads to brain damage, drugs that cause birth defects (thalidomide prescribed to treat morning sickness in pregnant women), sunlight & butterfly wing color