Single Gene Inheritance How Traits are Passed On

Single Gene Inheritance How Traits are Passed On

2 Genetics Units Heredity/Classical Genetics n Inheritance patterns n Based on the work of Gregor Mendel in the mid 1800 s Molecular Genetics n DNA/Chromosomal explanations for inheritance patterns n Defines our modern understandings of genetics

*Note n We are covering the genetics of sexually reproducing mammals n Bacteria/Archaea and asexual eukaryotes are a little different

Human Chromosomes

So you have 2 alleles, which determines what you look like? ? n It depends. This is where genetics starts to get complicated n Sometimes only 1 of the 2 alleles gets expressed n Sometimes themselves they both express

Case 1 – Complete Dominance n Only 1 trait is expressed n The dominant trait OR the recessive trait is expressed

Human Examples n You have dimples or you don’t n You have freckles or you don’t n You are a dwarf or you aren’t n You have attached earlobes or you don’t n You have cystic fibrosis or you don’t

Complete Dominance Dominant Trait n Is expressed if it is present n You can have 1 or 2 dominant alleles n Represented capital letters by Recessive Trait n Is expressed only if there is no dominant allele n You must have 2 recessive alleles (usually) n Represented by lower case letters

What’s Actually Happening? n One version codes for a functional protein, the other makes nothing n If one set of instructions makes enough proteins for the trait to be present, that trait is “dominant” n If one set of instructions does not make enough proteins for the trait to be present, that trait is “recessive”

The Dominant Phenotype n Phenotype = appearance n Is shown by individuals who have 1 or 2 dominant alleles n Called homozygous dominant (AA) or heterozygous (Aa) genotypes

The Recessive Phenotype n. Is only shown if the individual is homozygous recessive (aa)

“Genotypes” – Combinations of Alleles = homozygous dominant (shows dominant trait) =heterozygous (shows dominant trait) = homozygous recessive (shows recessive trait)

Punnett Square Dad Parent 1 Potential eggs Parent 2 Mom Potential sperm n Show the possible alleles each parent could give. n This Potential zygotes example shows an Aa (heterozygous) person mating with an aa (homozygous recessive individual)

�Say A codes for green gree eyes a codes for black eyes

Draw a Punnett Square for Two Heterozygotes Mating

Case 2 n If 2 different alleles are both present, their affects are combined n i. e. red allele + white allele pink flowers n Straight + curly hair Wavy hair

“Genotypes” – Incomplete Dominance = AA (red flowers, or straight hair) =Aa (pink flowers, or wavy hair) = aa (white flowers, or curly hair)

Incomplete Dominance n R= red allele, r= white allele n RR = red flowers, Rr=pink flowers, rr= white flowers n If red flowers are mated with white flowers, what will the offspring be? r r R Rr Rr

What’s actually happening? n One set of instructions produces some proteins, but the fewer proteins cause a change in the phenotype

Draw a Punnett Square for 2 pink flowers being crossed

Case 3 n. Both alleles are expressed at the same time and do not mix/form intermediate

Codominance n E. g. blood types n IA and IB alleles are both dominant n There is a 3 rd, recessive allele called i (codes for blood type O)

Blood Types n 4 possible types n. A (IAIA or IAi individuals) n B (IBIB or IBi) individuals n AB (IAIB individuals) n O (ii individuals)

What’s Actually Happening n Both sets of instructions code for functional proteins

What parents could have children with type A or type B blood only?

Summary n You have 2 copies of every gene, one from your mom, one from your dad n Which version you express depends on the gene, sometimes only 1 (complete dominance), sometimes a mixture (incomplete dominance) and sometimes both (co-dominant) n The “traits” are due to which protein is produced

X-Linked Genes n The X chromosome contains many non-sex related genes n E. g. an important gene for color vision- without individuals suffer color blindness n Since males have no second X chromosome, they express whatever allele is present on their single X chromosome n Punnett Squares are drawn with XA or Xa to show dominant/recessive traits and Y to represent the Y chromosome

X-Linked Punnett Square Eggs Sperms XA Xa XA X AX a Y X AY X a. Y All daughters show dominant phenotype Half of sons show recessive phenotype

Examples of X-linked genes in humans n. Hemophilia n. Color-blindness n. Duchenne’s Dystrophy Muscular

What has to happen for a girl to be color blind? (draw the square)

Pedigrees

Pedigree Take 2

Sex-linked Pedigree

What if the data doesn’t fit these patterns?

Possibility 1– the Trait is Polygenic n Multiple genes play a role in determining phenotype n Skin color n Height n Eye color n Hair color

Possibility 2– Environmental Effects n Environment plays huge role in gene expression n E. g. weight, strength n May also directly affect phenotype n E. g. hydrangeas

Possibility 3 - Epistasis n Another gene affects the expression of this gene n E. g. mice need to have a “color gene” to express the agouti or black color