Darwin's difficulties, genetics & inheritance

When Darwin’s postulates hold…

natural selection produces adaptation

• but Darwin didn’t know how it worked

What Darwin didn’t know (but his theory demanded)

1. Where does variation come from?

2. What maintains variation?

3. How do organisms come to resemble their parent?

4. How can natural selection extend the range of variation

The blending problem

Darwin and others believed that each parent contributed to offspring and these contributions were halved each generation (this is not the case, however)

How do we resolve Darwin’s difficulties?

We need to understand the nature of inheritance through Gregor Mendel’s studies

• Mendel’s research question: how are traits inherited from one generation to the next?

Mendel’s experiments

• focused on traits that have 2 forms

  • yellow and green seeds

  • smooth and wrinkled seeds

• cultivated true breeding and studied results of crosses

• crossed green and yellow parents

  • all offspring were yellow

  • crossed offspring (3:1 ratio)

Mendel induced two rules

1. characteristics of organisms are determined jointly by particles inherited from each parent

2. particles from each parent are equally likely to be transmitted to offspring

brief genetics overview

• 2 sets of genomes with 23 pairs of chromosomes

  • humans have 46 total chromosomes (23 from mom, 23 from dad)

  • homologous chromosome: pair (1 from mom, 1 from dad)

    • same genes in same location, though they can have different alleles

• haploid = 1 set

  • sex cells

• diploid = 2 sets

• sex chromosome: x and y

• autosome: all chromosomes (22 pairs) except sex chromosomes

key concept

• the relationship between DNA, RNA, and protein is that DNA is the template for RNA, and RNA is the template for making proteins

  • each chromosome is one pair of very long DNA molecules

DNA

what genes are made of

genes

section of DNA with identifiable structure or function

What is a chromosome

genes, DNA, alleles

alleles

different forms of a gene

protein synthesis

requires transcription and translation

What is key to Mendel’s laws

modern genetics

• cell contains nucleus

• nucleus contains chromosomes

• chromosomes come in pairs

• when somatic cells are divided, chromosomes are doubled

• each daughter cell has same complement has original cell

gametes vs. somatic cells

• gametes (egg and sperm) only have 1 copy of each chromosome

• when gametes fuse, they create a zygote

  • this fits Mendel’s laws because characteristics of organisms are determined jointly by particles inherited from each parent

  • Mendel’s particles = genes

In Mendel’s experiment…

• yellow parents produce gametes with yellow alleles, and green parents will produce gametes with green alleles

  • offspring inherit one gamete from each parent

phenotype

organism’s observable and measurable set of characteristics

genotype

organism’s complete set of genetic material

• for an offspring, genetic composition is not the same as either parent

more outcomes of Mendel’s experiment

• different traits assort independently

  • wrinkle vs. smooth, green vs. yellow

• genes on the same chromosome are linked

• genes on the same chromosome can recombine, but the rate will not be high

  • the closer the two loci are, the more likely they are to stay together

linkage

genes tend to stay together on the same chromosome