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Dihybrid inheritance is taught in the note for Inheritance. Read it below and look for the section on this topic.

Inheritance

16 min read

Hereditary materials

The hereditary material is DNA. Experiments by Griffith, and Avery, MacLeod and McCarty, showed that DNA carries the genetic information, and Watson and Crick (with Franklin’s data) described its double helix. A DNA nucleotide has a sugar (deoxyribose), a phosphate and one of four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). A pairs with T, and C pairs with G.

The structure of DNATwo long strands wound round each other in a double helix. Each strand has a backbone of sugar and phosphate. The strands are joined by pairs of bases: adenine always pairs with thymine and cytosine always pairs with guanine. The order of the bases along a strand is the genetic code.Base pairsA pairs with TC pairs with GA adenine T thymineC cytosine G guaninebackbone: sugar + phosphate

DNA replication

Before a cell divides, DNA is copied by semi-conservative replication. The enzyme helicase unzips the two strands, and each strand is used as a template. DNA polymerase adds complementary nucleotides. The result is two identical molecules, each with one old strand and one new strand.

Protein synthesis

A gene is a section of DNA that codes for a protein. Transcription makes a copy of the gene as messenger RNA (mRNA) in the nucleus, using uracil (U) instead of thymine. The mRNA moves to a ribosome, where translation takes place: transfer RNA (tRNA) molecules bring amino acids matching each three-base codon, and the amino acids are joined into a protein.

From DNA to proteinIn the nucleus an enzyme copies a gene on DNA into messenger RNA, a step called transcription. The mRNA leaves the nucleus and attaches to a ribosome in the cytoplasm. Transfer RNA molecules carrying amino acids read the code three bases at a time, and the amino acids are joined into a protein, a step called translation.DNA (gene)transcriptionmRNAto ribosomeRibosomeProteintranslation: tRNA brings amino acidscodon: 3 bases code for one amino acide.g. AUG = methionine (start)in the nucleusDNA → mRNA → protein
StepWhereWhat
TranscriptionNucleusDNA → mRNA
TranslationRibosomemRNA codons → amino acid chain

Dihybrid inheritance

When two genes on different chromosomes are inherited together, they assort independently (Mendel’s second law). A cross between two plants that are heterozygous for both seed shape and seed colour (RrYy × RrYy) gives a 9 : 3 : 3 : 1 ratio.

A dihybrid crossTwo genes inherited together: seed shape, round R or wrinkled r, and seed colour, yellow Y or green y. Crossing two RrYy plants gives sixteen combinations. Nine are round and yellow, three round and green, three wrinkled and yellow, and one wrinkled and green, a 9 to 3 to 3 to 1 ratio.RYRYRyRyrYrYryryRRYYRRYyRrYYRrYyRRYyRRyyRrYyRryyRrYYRrYyrrYYrrYyRrYyRryyrrYyrryy9 round, yellow3 round, green3 wrinkled, yellow1 wrinkled, greenRrYy × RrYy
Out of 16: nine round-yellow, three round-green, three wrinkled-yellow, one wrinkled-green.
Worked exampleA dihybrid cross

In 1 600 offspring of RrYy × RrYy plants, how many do you expect to be wrinkled and green?

  1. Wrinkled and green is rryy, which is 1 in 16.

Answer: 100 plants.

Try itExpected numbers in a dihybrid cross

Choose the number of offspring and see the expected numbers in each class.

Check yourself In DNA, adenine pairs with...
Check yourself Where does translation happen?
Check yourself The phenotypic ratio from RrYy × RrYy is...

Key terms

Replication
The copying of DNA.
Transcription
Copying a gene into mRNA.
Translation
Making a protein from mRNA.
Codon
Three bases that code for an amino acid.

Summary

  • DNA is the hereditary material; A pairs with T and C pairs with G.
  • Replication is semi-conservative.
  • Protein synthesis: DNA → mRNA (transcription) → protein (translation).
  • A dihybrid cross gives 9 : 3 : 3 : 1.
Protein synthesis