2.5: principles of heredity (genetics)

Chapter 6: principles of heredity (genetics)

What carries genetic information in most organisms?

Chromosomes

Most organisms have corresponding pairs of genes for the same traits known as ____

homologous chromosomes; Genes that produce a given trait exist at the same position (or locus) on homologous chromosomes

Alleles

Different forms of genes; A gene can have 2 or more alleles, which differ in their nucleotide sequence; that difference can translate into proteins that function differently resulting in variations of the trait; Ex: yellow seeds and green seeds arise from different alleles of the same gene 

What does meiosis produce?

produces gamete cells with ½ the genetic information of the parents (paired chromosomes are separated and sorted independently) ; Each gamete may receive any number of combinations of each parents chromosomes

A trait may arise from ___

1 or more genes

If a trait is ____, several possibilities for traits exist

produced from a gene or genes with varying alleles,

Genotype

the combination of alleles that make a particular trait

Phenotype

trait expressed

An allele is considered dominant if ___

it masks the effect of its partner allele (recessive allele)

Punnet square

notation that helps predict results of genetic crossing; A letter is assigned to each gene; Uppercase letters represent dominant genes, while lowercase represent recessive genes

homozygous

When both alleles for a given gene are in the same individual (such as YY or yy)

Heterozygous

When 2 alleles for a given gene are different in an individual

Monohybrid cross

 a cross between 2 individuals where only 1 trait is considered

Dihybrid cross

cross between 2 individuals where 2 separate traits are considered

The law of segregation

a diploid organism has two alleles for each trait, and these alleles separate randomly during gamete (sperm and egg) formation. Consequently, each gamete receives only one allele, ensuring that offspring inherit exactly one allele from each parent.

The law of dominance

The gene usually expressed over the other was the dominant gene

The law of independent assortment

• Whether genes for 1 trait were always linked to genes for another 

• Determined that most traits were independent of one another 

• In most cases genes for traits randomly sort into pairs (although some genes lie close to others on a chromosome and can therefore be inherited together) 

Incomplete dominance

• Some traits are determined by genes that are neither dominant or recessige & instead produce offspring that are a mix of the 2 parents 

• Conventional way to symbolize the alleles with with a capital letter designating the trait (ex: C for color) & a superscript designating the allele choices (in this case R for red, W for white) 

Multiple alleles

There are some instances where more than 2 choices of alleles are present; Ex: blood types

Linkage

• While Mendel had established the law of independent assortment, later study of genetics found that this law was not always true

• Some traits are always inherited together rather than being independently sorted 

• Traits inherited together are said to be linked

• Most traits produced by genes that are close together on the same chromosome are inherited together 

• The chromosomes are independently sorted, not the individual genes 

• Crossing over (during Meiosis I) adds even more possibility of variation of traits among species 

  • It is more likely for crossing over to occur between genes that do not life close together on a chromosome than those that lie close together 

How is gender determined in an organism?

by a particular homologous pair of chromosomes ; X & Y denote sex chromosomes; In mammals and many insects the male as XY & the female has XX

Sex linked trait

• more males develop the trait because males only have 1 copy of the X chromosome

  • Females have a second X gene which may carry a gene coding for a functional protein for the trait in question that may counteract a recessive trait 

  • Ex: colorblindness & hemophilia 

Sex-influenced traits

• trait is autosomal - it only requires 1 recessive gene to be expressed if there is no counteracting dominant gene 

  • A male with 1 recessive allele will develop the trait, whereas a female would require 2 recessive genes to develop it 

  • Ex: male pattern baldness 

Polygenic inheritance

Most traits, such as height and skin color, are produced from the expression of more than 1 set of genes; Traits produced from interaction of multiple sets of genes are known as polygenic traits