HL Bio - Unit 2: Monohybrid Crosses

Inheritance in plants

Pollen is transferred from another male plants anther to the stigma of the the female parent

Pollen germinates and the gametes are carried down to the plants ovaries

The zygotes that are formed by the fusion inside the ovules will develop into embryos within seeds

Artificial inheritance

done by just brushing the pollen on the stigma and cutting off all other anthers to prevent pollen to transfer from them as well

Notation for generations

• Parents: P generation

• First filial generation: F₁ 

• Offsprings of F₁ or second filial generation: F₂

Gregor Mendel’s

(Father of genetics): He did crosses between peas to reveal basic inheritance - crossed tall plants with dwarf or white with purple etc

• He created the punnetts grid: used to analyze the result of genetic crosses

Genotypes

Combinations of alleles

Alleles

• Form different versions of genes - they can differ by just one base in the sequence 

  • New alleles are only generated through mutations

Homozygous Genotype

where both are the same allele

Heterozygous Genotype

Where there are different alleles - normally one recessive and one dominant

Dominant vs Recessive Alleles

• Dominant alleles will overshadow any recessive ones 

• Symbols

  • Lowercase letters are used for recessive alleles

  • Uppercase letters are used for dominant alleles

Two dominant alleles together could be expressed in two different ways

• Incomplete Dominance

• Codominance

Incomplete Dominance

• Blended - They will form some halfway point 

  • heterozygous having an intermediate phenotype (blend)

  • FOr example, Mirabilis jalapa: A plant than can be both white or red and if both the dominate red and white alleles are present it will produce a pink flower 

Codominance

Both expressed at the same time - like the spots on a roan cow

Carrier

• an individual who has a recessive allele of a gene that does not have an affect on the phenotype due to a dominant alleles expression

Phenotype

Observable traits of an organism caused by their genotype

• Includes structural characteristics or functional traitss

Observable

Trait is visible or can be detected with tests

Genotype Only:

• Eye color 

• Haemophilia: Lack of ability to form blood clots

• Ability to smell B-ionone: Odor in violet

Environment and Genotype:

• Height

• Autism - personality trait

• Diabetes 

Environment Only:

• Scars

• River blindness (onchocerciasis) 

• Body art

Phenotypic Plasticity

The ability to vary their gene expression patterns by turning genes on or off (eg, skin color in more sun exposure regions)

Polypeptide and recessive alleles

Produced through mutations that alter the sequence of a polypeptide - causes them to be less effective 

• Example: if the polypeptide is an enzyme some can still be normal and those amounts could still work well enough for no significant difference

Monohybrid Crosses

• Involve only one gene - both purebred parents causing only one type of gamete

Phenylketonuria (PKU)

Recessive allele: Caused by mutations in the autosomal PAH gene that results in low levels of the enzyme phenylalanine hydroxylase. 

• Phenylalanine hydroxylase (enzyme) converts the amino acid phenylalanine into tyrosine (non toxic)

  • phenylalanine can become toxic if there is an abundance of protein in a child's diet resulting in high levels of the amino acid and impaired brain development . 

• Children diagnosed with PKU are recommended a diet that omits food rich in Phenylalanine (eggs, chicken, nuts) 

Only way to get recessive diseases

When both parents are carries or have the disease

Autosomal

• Any genes that aren’t related to sex - aren’t on the sex chromosomes